A 10 tooth sprocket does several things. It primarily helps those of us who are less skilled with the clutch. You can go slower with the clutch out so it requires less clutch work. If you do a series of very close obstacles requiring a burst of throttle, it is easier to keep your speed down. (think - log, log, log) If the throttle does get away from you, the bike runs out of rpm sooner so is less likely to squirt out from under you. A 9 is even better for these things but made it harder to get over larger obstacles so not desirable. It felt like it took away the bikes reach.