Hnt/Fsh,

When you're on the bank, you're fishing from a fixed position. When your lead makes contact with the bottom, your corky/bait gets in line with the lead as it goes downstream. This is the typical driftfishing technique. Sometimes a longer leader is needed to get your presentation away from the lead, but there's really no need for a leader longer than 4 or 5 feet.

The flossing technique is different. You cast out that long-ass leader, and when your lead makes contact with the bottom, you start reeling in line so you sweep that long leader across the bottom.....and if fish are stacked in there, you're going to hook one whether they're biting or not.

When you sidedrift from a boat the boat stays in motion, so the long leader combined with the moving boat gives the corky/bait a gliding, natural drift. It's a completely different presentation than you get from the bank, and most of the top guides who sidedrift use 6ft +/- leaders.

I just can't fathom the reason for using a 10 foot leader from the bank aside from it's effectiveness for flossing. Of course, you don't need a 10 foot leader to floss on a smaller river, either. It's the cast, wait, reel in slowly and steadily, and set the hook if the line comes tight presentation that identifies the flosser. Some guys are really good at it, but I never found much sport in it myself.