These are some of the xingyiquan forms that we practice. Comments mix information from the "Xingyiquan Encyclopedia" (形意拳術大全) published by the Shanxi People's Publishing House (main editor: Wu Dianke 吳殿科) and my own observations and experiences.
In Li Luoneng's time, this form was known as the "5 Element Linking Fist" (wu xing lian huan quan, 五行連環拳). After Che Yizhai's compliation and editing of the forms, it became known by it's current name. When practicing this form, the key idea to keep in mind is that "within advancing there is retreating and within retreating there is advancing". That is, even when advancing you need to be able to retreat instantly, and while retreating, you need to be able to instantly change to advancing.
This form has undergone several changes in the last hundred years and it's possible to see several variations done by modern practictioners. For instance, in "One Horse, Three Arrows" (yi ma san jian, 一馬三箭) some practitioners go into a deep cross legged stance on the first two bengs, while others only cross on the retreat. Another interesting variation doesn't use a crossed stance at all - the first beng (崩) is done as a half step beng, and the second beng (on the retreating step) hits just as the rear heel roots into the ground, allowing for a very quick third (advancing) beng.
[The original Dai style version of yi ma san jian also has three strikes, but there is no retreating step. The first strike is a cross step (ao bu, 拗步) punch. The second one is another ao bu strike, this time with the other hand as the rear leg pulls in and forward to assume the "squatting monkey" (dun hou, 蹲猴) posture. The final strike is with the (now) lead hand as the rear leg continues to step forward into a "bow stance" (gong bu, 弓步). The first two strikes hit while the body contracts (shu, 束), and the third one strikes on the expand (zhan, 展). Apparently the retreating step strike is a modification by Li Luoneng or Che Yizhai.]
Essential points:
Che Yizhai derived this form from Dai style's "Five Section Watergate Posture" form (wu tang zha shi, 五趟閘勢), also known as the "Preying Mantis Watergate Posture" form (tang lang zha shi, 螳螂閘勢). The "Watergate Posture" (zha shi, 閘勢) is the origin of the "Cat Washes Face" (mao er xi lian, 貓兒洗臉) movement. The tang lang zha shi form is usually done as five separate sections (wu tang, 五趟). Each section ends with zha shi, and you can see the same separations (in the form of "cat washes face") in the za shi chui.
Even though za shi chui is derived from the tang lang zha shi form, the power generation is not quite the same. The za shi chui version also has more fist techniques. Since the form incorporates a variety of postures and uses a lot of fists, it was named "Mixed Posture Pounding".