Simplest answer is when the gearing combo you choose is trying to turn way more RPMS than the engine will turn, an example would be your at a fifth mile momentum track with a clone targeting 6800 rpm and a 16 /60 is perfect gearing combo and you started on a 16 / 66 rpms were low, so you chase it even more and go 68 rear, that's gear bound think of it as your engine is running with it's tonge hanging out the entire race lol. !!
I was running a big track this past weekend (5/16 mile) running a 17/68 turning 6200rpm. I needed more top end speed so I went 17/65 but for some reason I couldn't get the rpm's up, they dropped to 5300. Should I have gone the other direction on the rear or should I have dropped the driver to a 16? Why? I'm new to all of this just my 3rd weekend in a clone.