A soft edge is where you never let yourself get close enough that you could cum by accident – it’s useful if you keep going over when you don’t want to. Whereas a hard edge is trying to get as close as you can without going over, teetering right on the brink.
Interestingly, while the immediate impact of a hard edge is obviously stronger, over time, soft edges can get you just as horny as hard edges. Although they can drive you a bit crazy as you never have an ‘end point’ that at least partially satisfies you.