Hand/foot timing is stressed to beginning students, and even black belts must be reminded periodically. Hand/foot timing is when, as you are stepping into a hand technique (attack or block), the hand reaches the target at the same movement the foot touches the floor. Arm and leg motions stop simultaneously (usually the hip's rotation and overall momentum of the body also stops at this point). If the hand reaches the target before or after foot contact, power will be greatly reduced. For maximum power, both must touch simultaneously.
However, there are advantages to mistiming. The foot landing early may provide the punch with more stability. The foot landing late may absorb recoil after the punch connects.
Usually more power is lost when punching while stepping backward than when stepping forward, if you punch before the foot lands. In contrast, punching after the foot lands seems to work well when stepping backward. This appears to be a result of your momentum moving backward while you are punching forward.
Hand/foot timing is especially critical with performing patterns. Since you are moving a slower pace than when sparring and since judges know each movement you are performing, they are sensitive to any hand/foot mistiming. Proper chambering before each technique aids ensures hand/foot timing during patterns.