Hitting a huge shot, grabbing a rebound, getting a steal, all are very big aspects of getting the momentum. But none can get your team fired up like a huge shot block. A blocked shot really takes the wind out of a team that is doing well. Having a few shot blockers on a team makes the opposition fearful to drive inside, and makes them a one-dimensional team by only taking perimiter shots.
To be a good shot-blocker, though, you must have some of four things. Height, long arms, the ability to jump high and instinct. To get around your block, the person with the ball will try a number of things. If he trys a fade-away jumpshot, jump diagonal in the direction that person is shooting. Look at the person's waist while defending him/her so you don't bite on a head fake. If a person tries a hook-shot, go around him so you take away his (hook) shot. Following these rules will make you a better shot-blocker.