Q: "what do rear disks do to frames?" A: Err, in the shoddier designed frames a rear disk can break them? Q: "two, What is modulation?" A: Modulation is basically the slowing down effect of brakes, the more modulation in a brake, the less likely they are to lock up. e.g, An hs33 with Plazmatics, a really savage grind, and a really good booster would give you little or no modulation, the brake would feel very off and on...perfect for a trials rear brake... however an hs33 with, say, koolstops, an ok booster and no grind would mdulate and slow you down really well....and wouldn't be very grabby...perfect for manuals and stoppies