The technique Zhāi Guā (摘瓜) literally means plucking a melon from the ground, with the melon being the opponent’s head. This is an interesting move not because it is particularly effective, which it is, but for the clever ways of guiding away the opponent’s guard and arriving at the position to with body movement.
The trick here is to put pressure on the opponent so he would give you counter pressure. This is an important concept because: 1) when facing a competent opponent, the chances are good that he would be able to parry or move away from your initial attack, so he would not suffer much even if the hit connected; 2) for the above reason, it is risky to throw a heavy punch on the first move, as it would slow you down and extend the tempo, thus creating a larger time gap between you and the opponent, which he could use to either move away or counter you.
The Mantis School answers the twin questions in two steps. First, we initiating the attack by presenting a thrust to the eyes with your arms moving in the ways of the Double Closure. This attack serves two purposes: 1) it ensures the attack can be conducted with maximum damage and speed; 2) the opponent would be forced to react to it even if it fails to connect. Either way, the goal is to gain initiative.
Should the opponent is forced to react to your eye thrust with his left, to perform this particular move, you need to follow his resistance, draw a circle from your right to left. This move allows you to misdirect the opponent as he will feel that he has successfully parried your attack and may consider him being safe. Your real target is, however, his right arm, which you will reach from beneath by continuing the circle.
Once you have reached his arm, yank his body towards you with a reverse pull, and at the same time, shoot your body forward crossing the opponent’s body from behind; either one of you arm should also shoot forward as well to prepare for the lock. The yank serves to upset his balance, while giving you the space required to get to his back. Once you are squarely behind him, you have reached the ideal position to perform the headlock. If the opponent continue to resist hard against the lock, or if he is taller then you expected, kick either one of joints of his legs from the back to remove his balance from under and give it an extra hanging effect.
Suppose the opponent tries to counter attack with his right during the initial contact, you can then try a variation of the move by guiding his arm upward so you have the space to duck and shoot your body through under his arm or armpit. In the midst of the duck, you need to shoot your arm upward to reach the left side of his neck, and perform the lock from the side, with his arm lifted up and in between you and his face.
Suppose you cannot move your body under his arm due to him blocking the route, you can simply try to shoot just your arm underneath then lock his shoulder to perform some variants of Off-Helmet (摘盔) or Sweep (掃邊手).
If he does not react in the manner so you can perform any of the above, then you simply do something else. Remember, the goal is to take the initiative and control the fight.