You really need to be Yemenite for this, and pronounce the ches and ayin the way they still do, in the back of the throat. (Rishonim call these letters גרוני, "pronounced in the throat". HHes (really they say HHHeth), rasping in the back of the throat. And AAyin, in the same place.
What's the difference between them? You hum when you say an Ayin - "voiced". Ches is "unvoiced". Sometimes in English we hear the difference - V and F - and sometimes we don't even notice, as in the th of "this" and "theater" (say them out loud.)
So an ayin is a lot like a ches, only it's got that extra measure of energy from being voiced.
Note that there are two types of sh'va: a sh'va na (נע), which is voiced, and a sh'va nach (נח), which is silent.
Why am I bothering you with this? Here are two gemaras:
Sanhedrin 103a, "Menashe the king of Yehudah in his great suffering in exile wanted to do teshuvah, and prayed to Hashem. And Hashem heard ויחתר לו. Says the gemara, it should say ויעתר לו, Hashem heard his prayer. But it's teaching us that Hashem had to make a tunnel (מחתרת) in the heavens to receive his teshuvah."
Now the gemara in Sukkah 14a says that the prayers of tzaddikim are like an עתר, a pitchfork. They flip the din right over from top to bottom. With an ayin, that's a very active thing to do to the sins.
Menashe wasn't able to do that. He was in a bad way, with enormously heavy sins; they couldn't be flipped so easily. He had to use a ches, less active, leaving the sins in place, so to speak: he tunnelled around them.
Another gemara with the same theme: Arachin 15b. "A tongue that speaks lashon hara is deadly - as it is written, חץ שחוט לשונם, their tongues are arrows of death. And what's the cure? Torah, which gives life, as it is written, עץ חיים היא, it is a tree of life."
Same tongue, but: An arrow, with a ches, is death. A tree, with an ayin in place of the ches, is life. חץ - עץ.
And an arrow is just a dead tree!
[By the way, there are only two Hebrew letters that look like trees, with a single root below and branches above: ע ץ ]