Consider ABCD be a parallelogram. The parallelogram law says that:
If you learn the construction of a bow with an arrow, you will notice that its form is similar to parallelogram.
Its sides AB, BC, CD and AD are constant. When you are going to shoot, you pull the arrow, so the diagonal AC becomes longer. Due to the parallelogram law, as the left part of the equation remains the same, the diagonal BD becomes shorter. So the bow becomes bent and the pressure on the arrow grows. All that you need is to release the arrow and make a shot.