A ball detent keeps the ball that you had drop down from your Revo most recently from moving forward so that another ball doesn't feed halfway or completely into the barrel, causing you either to chop, send two balls (which would be illegal according to tourney rules, because one ball per pull is all that is allowed) downrange which neither would be likely to hit anything, blow one or both up in your barrel, or do other weird stuff, none of which you would likely want to happen.