So this SSFIV OS is stupidly good: http://www.eventhubs.com/news/2013/nov/17/proximity-blocking-option-selects-punish-whiffs-and-more-super-street-fighter-4-arcade-edition-v2012/ James Chen said he received word that it's training mode only, but some people are saying they've done it in versus mode. Basically it seems like proximity blocking either extends the command input buffer by several frames, or it shortens a moves recovery. So if you buffer a move during the recovery of another and immediately hold downback, the buffered move will only come out if the opponent whiffs a move a range that activates proximity blocking. I haven't tested it myself though.