Never leave your feet on defense -- not even to block a pass, not even to block a shot. For every pass you block by jumping, two will get by you and you will be slow getting to good defensive help-position. You have to wait until you come down to move. For every shot you block, twice you will foul unnecessarily and another time or two the shooter will miss but be able to run by you for the rebound, since you cannot block out in the air. Jumping is not a valuable skill on defense (unless you are a gifted, intimidating center). Don't jump. Never leave your feet. Keep your feet on the floor at all times on defense.