Cai, as "pluck" or "pull," usually requires a degree of sticking or grabbing.
In the steps, Cai may be applied with the foot or ankle. For example, in Wave Hands, the right foot pulls in to follow the sideways shift of the body. To become Cai, the foot hooks behind the opponent's ankle, and uses the shifting weight and closing step to pull the opponent's leg. To do this, the opponent is standing in a forward stance, while you step away sideways, either to the side or along his line.
Another example is in the backward breaking step (Repulse Monkey), where you hook behind the opponent's lead foot and step back. This will pull his weight abruptly forward (ideally into your palm strike; practise this without hands at first).
In a raised-leg posture (Golden Rooster) this same scenario is present. As you raise your knee to ward off the opponent's knee (directed to his inner thigh), hook your calf around his and step backward.
The most difficult expression of this energy comes in one of the stranger applications of the kicks, and is an excellent demonstration of "legs against legs." As the opponent kicks, use your kick not to kick him, but to ward off his leg (again, against the inside of his leg). Your leg then wraps over the top of his leg. Use the recoil of your kick to trap or "grab" the opponent's lower leg, then step back. This move is extremely difficult. I've only pulled it off once myself, but that was in sparring with a Tae Kwon Do fighter, so I know it works. I will say, however, that it is much easier to do fast than slow.
SB