The saying "Receive What Comes," or "If the Attack Comes, then Meet it," means that if the opponent initiates the attack then you should meet the attack with an attack response of your own.
For example, if the opponent throws a straight punch, then meet the punch with a thrusting hand or Bil sau movement directed towards the opponent's center. Alternatively, you can apply a palm up hand (Tan sau) and punch.
If you immediately counterattack, then the opponent risks getting hit. This places him on the defensive and slows down his attacking action.
By meeting the opponent's attack, you are simply engaging the opponent's force so that you can determine the next actions based on what you feel. When you feel the opponent's force, you can tell if it is strong, weak, stiff, sticky, soft, centered, off-centered, etc. Your next action depends on what you feel.
The saying "Follow what goes," means that if the opponent withdraws his force, then you must stick to the opponent and hit. As long as you are sticking with the opponent, you are unlikely to lose.
If you do not stick with the opponent, then you have lost an important indicator of what will happen next. The indicator is your sense of touch. Upon loss of contact, the opponent can kick or perform many kinds of deceptive hand techniques which are bound to succeed.
If the opponent retreats the stance, then you rush in with your stance. If you just stand there, then you will get kicked. If you decide to retreat, then you must again close the gap between you and the opponent. Since this part is less scientific than sticking hands fighting, you risk getting hurt in the clash.
In the sticking hands training, you can practice retreating while your partner chases and sticks.
The saying, "Rush in upon loss of contact," means to hit straight if arm contact disappears. Imagine being blindfolded. Your hands are in contact with the opponent's hands. You can detect and feel every movement that the opponent is about to make. Now imagine that there is a loss of hand contact. You do not know where the opponent's hands are. You have no sensory clues about what is going to happen next. You are bound to get hit.
Close range fighting is often like the blindfolded case. Your visual system can't always see what is going on because the action it too rapid or the vision is blocked (the action is too close). If you maintain a slight forward pressure directly towards the center line of the opponent and strike in a straight line upon loss of hand contact, then you will hit the opponent first. The reason is that a straight line is shorter than a curved line. If you are in the center, the opponent must travel a curved line to disengage.
The saying "Make the first move to have control," is also expressed as "If he stays I go." What these expressions mean is that, if the opponent is facing you and does nothing, then don't hesitate, but attack right away. Otherwise, you may be tricked by an opponent who has figured you out. The intention of your attack is to make contact with the opponent. Once you have made contact, then you can use your knowledge from the sticking hands to win.
What the above saying does not mean is to just rush in recklessly without fear of getting hurt. If you are weaker than your opponent or slower, or if you don't know anything abut how your opponent fights, then you stand to get hit. In Wing Chun, advance slowly and cautiously toward the opponent, always threatening him. At the right moment, suddenly charge. The right moment may occur during an attention lapse or during the time an opponent tenses up, or at the completion of an opponent's movement. Lift the leg immediately against a low kick.
The saying "Attack according to timing," is meant as a caution to not just blindly rush in.
The Wing Chun idea of how to attack is like a cat trying to get a mouse. The cat sneaks up very slowly to the mouse. The closer the cat gets, the more careful the cat gets. Once the cat is close enough, then the cat does an explosive forward springing action to get the mouse.
The Wing Chun Kung Fu Digital Library