You hold the weight in the contracted position. For example for back if you're doing palms up seated rows with a cable, you hold the weight close to you for a determined amount of time.
max contraction is a bodybuilding term that means you lift a weight to the point where the targeted muscle is contracted at its highest. take biceps curls for example...if you curl too high and your elbows start coming forward too far (a little is ok)...your front delts come into play and your PAST the point of max contraction. its the point where you flex the muscle and if you flex any more...another muscle has to help out.