You won't know where the line is until you cross it...?  Or at least, come close to it.  Best way to find the edge is to brush up against it.

It's funny...I feel like it's something I do so often and so second-naturedly in video games...I've complained before that learning how to drive is made really difficult by the fact that the penalty for screwing up is so high, so you're too afraid.  You overcompensate in one direction, so you can't just nail things in the middle.  Or something.  Well, I guess the real reason that driving is hard is because you don't have an overhead camera.  But I digress...