Good defense is often the combination of positioning and athleticism. Man-to-man, pressure defense can be played many ways, and today we are illustrating defending away from the ball, and contesting rather than denying. Here, with the ball at the top (arrow) and in Loyola Marymount's possession (vs St. Mary's), there is an LMU player between the ball and James Walker’s man (Walker is circled, and his man is in the corner). This is referred to as being two passes away, as in order to get it to Walker’s man it would either take two passes or one long skip pass, in which time Walker would have the ability to get there. So Walker is cheating underneath where he can help with any action in the middle.
Once the action (arrow) moves closer to his man, Walker is now "one pass away" which causes him to step up into proper help position. The line between the two players is the one that the ball would travel on a pass. Help (contest) position means he’s one big step off that line and cheated toward the ball. If he were in a denial position, he would be angled in a way that would allow him to have his hand extended into the passing lane.
LMU attempts a drive but can’t go anywhere because Walker is there to help, and then when the player spins out of it, he slips. Here, Walker’s length and speed allow him to get the steal. That's help defense. Spend some time looking for it in the next games you watch, and pretty soon you'll be able to recognize when a player is out of position. And once you can recognize that at game speed, you'll be smarter than most of your friends.