There are a lot of different things within physics that apply to the game of football. One of these is momentum. In football, its all about hitting the guy with the ball or just hitting the guy in front of you to make the block. A tip that most people say is that right before you get in the collision, you must relax your body to reduce injury. This also helps the shift in momentum because when you do this, you are allowing all the energy in your body to be transferred into the other player (Shift in the momentum). If you tense up, you are allowing some energy to be stored in your body which will lower the power of the hit.