"Left plastic" refers to those politicians or activists with a pseudo left wing ideology and/or a "Greater Chinese" ideology (see below). This does not refer to the pro-Beijing political organizations or figures, even though they are traditionally referred to as "on the left".
Left plastics are unique to HK politics. They are, at a superficial level, similar to the western "champagne socialists", but due to Chinese nationalism influences, hold curiously contradictory beliefs. Although they superficially subscribe to pro-western values e.g. democracy and human rights, and appear to be against the Chinese communist regime, many of them sympathize with the idea of communism and hope the communist party will "self-improve".
It is very difficult to give a precise definition of left plastics; its meaning even in Chinese is somewhat ambiguous. There is a narrower meaning and a broader meaning (see below).
Left plastic is generally a negative term. "Plastic" originates on-line at a forum, hkgolden.com, and sounds similar to (one of the many words that mean) "penis" in Cantonese, and this is where its derogatory meaning come from. However, due to its origin from the online forum which is largely for fun, "plastic" carries a more light-hearted sense, sometimes almost to the point of being affectionate.
Left plastics can be categorized very loosely into at least the following types (with most people belonging to more than one type):
(1) "Progressive" social activists
These are (usually young) activists who have read a lot of modern left wing ideologies. They interpret any HK-China tension in the context of class struggle, and believe that all such problems originate from the evil of capitalism. They believe Hongkongers carry the "original sin" of capitalism due to HK's British heritage, and therefore think that it is always HKers who are wrong and Chinese who are right. They tend to offer "alternative", counter-intuitive views on issues so as to be seen as highly educated and "different".
They are what the original, narrower definition of "left plastic" refers to. They are the "left" at the academic level.
(2) Social workers and grassroot supporters
These are people and organizations that support the grassroots, e.g. social workers. Since the Chinese are perceived as "poor", they believe HKers need to help them at all expense, for example by allowing them to immigrate and claim benefits. They are the "left" at the more material level.
(3) "Absentee middle-class"
These are usually middle-class people who emphasize western values e.g. democracy and human rights, but only when the consequences do not hurt them. They enjoy occupying the moral high ground, and do not feel the suffering that many HKers face every day as a result of the Chinese colonization, e.g. mass immigration and cross-border smuggling; but as soon as these policy hurt them they will turn 180 degrees. Typical "not in my backyard" type of people. Many have western passports and can leave HK anytime.
(4) "Greater-Chinese" plastic
They hold nationalist, patriotic views toward China and feel strongly that they are Chinese and love China, even though they claim what they love is the country and not the regime. They believe China must remain "unified" (so for example they are against any kind of separation, let alone independence, of HK), even though this means throwing millions of people into the ruthless dictatorial regime. They believe the prosperity that HK enjoyed, as a result of its separation from China during its turmoil in 1950s-70s, is an "original sin" and HK people must "pay back the debt".
They are not really "left", but since their actions lead to similar consequences as the left plastics, they are often grouped together.
General criticisms against left plastics include:
(A) Theory
(A.1) They claim they support "universal values" (i.e. western values), for example tolerance (towards Chinese immigrants and smugglers), but in fact the true reason behind is their nationalist views. For example, they support "family reunion" of Chinese immigrants without any screening that ensures they can financially support themselves - the current policy is that such screening is required for all other migrants (western, Indian/Pakistani, or even Taiwanese) into HK; only mainland Chinese is free from this requirement.
(A.2) They follow the western leftist tradition of self-hatred; they hate HK and believe HK has the original sin of being capitalist. The West has the so-called "white guilt" and (for example) deliberately accept many migrants so as to cause suffering and to "redeem" themselves from their imperial past. Obviously HK left plastics are not white people, yet they imagine themselves to be in the same position (because of their subconscious belief that they are "superior" to mainland Chinese, just like western leftists subconsciously believe they are "superior" to ethnic minorities. This despite they claim to be leftists and treat everyone equal, of course.) As a result they support, for example, unlimited immigration. "Greater Chinese plastics" hold a similar view though the "origin" of the "sin" is different (see above).
(A.3) "Abstractizing" any protest or movement: for example, in April 2013 port workers took strike action against low pay, and once the left plastics entered the picture, they turned the theme of the protest into against capitalism in general. They diverted the protests away from specific entities (e.g. individual employers) towards "the system". By raising the issues to such an "altitude" of broadness and generality, they spread a sense of helplessness among the average protestors, that they cannot fight against the government machine.
(B) Protest style
(B.1) Their continually-losing protests are often ridiculed as "sustainable". Even though they would not admit it, they actually want to remain in a losing position. For one, if they actually "win" then they become the strong and the dominant, and this is against the leftist tradition of supporting the weak and the suppressed. Secondly, being left plastics is actually a job, a career for many of them, and if the protestors succeed there is no more need for the left plastics. Hence their protests do not aim for success, but rather to lose in a dignified way. If somehow a success is getting close, they will sabotage the protests: using slogans like "we already obtained intermediate success" and "we need to spread this movement wider to the general public", they disband any gathered protestors. Often they hijack protests started by others to sabotage them.
(B.2) Unlike some of the more extreme leftists in the west, the left plastics in HK are obsessed with maintaining the "purity" of their protests and always occupy the moral high ground. They emphasize the need to remain peaceful, and condemn any protestors who took actions as tiny as throwing a bottle. This is also partly to attract larger crowds since most HKers remain unawakened and any "violent" actions are frowned upon by them. As a result, most of their protests follow the same "routine" every time, with "singing" being most ridiculed.
(B.3) Their protests involve demonstrating that "the people are weak" and beg for mercy from the ruthless dictatorial regime. They "sell" emotions and sufferings, e.g. by kneeling in heavy rain or hunger strikes (and their hunger strike is a joke - done in shifts!)
(B.4) Emphasize the need to "extend the imagination" of protests: instead of actually taking actions, they ask people to sit down and "think" and "discuss" the directions of the protests. Very often turning and diluting a spontaneous, (government-)provoked protests into a long, never-ending series of discussions.
(B.5) Patronizing other protestors: they feel they "need" to lead the protests, and "teach" the public how to protest, e.g. to remain non-violent.
(C) Personal integrity
(C.1) Middle class background: some of them do not protest because they really need to fight for something, but rather as a "lifestyle".
(C.2) Conflict of interest: many of them earn their living by being in the leftist sector, e.g. working in NGOs. For example, social workers support mainland Chinese immigration since only more "poor" people arriving will give them more work to do.
(C.3) Hypocrites: for example, they criticize other people for being non-tolerant towards the bad behaviour of Chinese tourists, but when they themselves were affected they are the non-tolerant ones. Most prominently in the South Horizon incident where they protest against the opening of a shopping outlet (that target Chinese tourists) near where they live.