Impact: Refers to the moment the strike makes contact with the opponent, it is used for attacking movements and blocks, all power is concentrated in that moment. It is the way to hit with enough power to stagger your opponent, whether with a kick or a punch.
Kicks and punches have to have the power of the whole body (the reaching and turning power from the elbow, knee, hips) behind them.
The Impact Zone of a Ushiro Mawashi Geri
This can be compared with baseball batting. You have to bring the bat (your fist, leg) to the ball (the opponent) at just the right place and at just the right time, and direct the entire weight of your body through that point and moment of impact. So it is very important to save your energy for that crucial moment.
In Ashihara Karate, we rely on explosive power to create impact. To this end, we do lots of training which involved Plyometrics, etc. to develop this.