1) It helps a defender block some shots, which reduces their chances of going in to zero.
2) It helps a defender alter some shots, which reduces their chances of going in substantially but not to zero." Here, I'm not sure what you exactly mean.Whenever a shot is attempted, the game engine does a probabilistic calculation to decide whether it goes in. A shot which is neither blocked nor altered might be calculated as having X chance to go in. A shot which is altered has less than X chance to go in (but greater than 0). A shot which is blocked has 0 chance to go in.
2) It helps a defender alter some shots, which reduces their chances of going in substantially but not to zero." Here, I'm not sure what you exactly mean.
is it a block when it goes in for the stats? Maybe is it a block and a own basket here, because the ball hitting the floor - but pretty funny scene.