Lets say that you are on your back day and rounding out your last set of pull ups like you should be doing and you cant pull yourself up one more time.  Instead of giving up and stopping the exercise, you hold in the bottom part of the movement like you are going to fall off a cliff.  It is things like this that will push ypour gains in the right direction.  This can make for some serious extremely tough workout.  But this is awesome and will make you feel super powerful.  Try some thing like this to get you toy the next level.