How Bands Work:

When we do a conventional resistance exercise, we are only as strong throughout the whole of the movement as we are at our weakest point (usually in the middle of the lift).  There is also a point near the top of the movement where our body makes the weight slow down in order to keep it under control, which can be 50% or more of the total lift at high percentages of a person's 1 repetition maximum.  However, the top of the lift is where we are strongest.  That means up to 50% of the movement is essentially wasted! 

Bands allow us to get more resistance at the top portion of the lift than at the middle or bottom, allowing our muscles and skeleton to be under maximal tension throughout more of the range of motion of the lift.  This means we are working our muscles harder with every repetition.  Because of this, there isn't the need for as many repetitions to be done, which decreases the risk of injury and over-training.