Bounce back passes were tickets that were only sold at the parks. the idea was to try to upsell you. If you balked at the idea of purchasing a 4-7 day Park Hopper, they would try to sell you additional days at a meager discount, OR if after you got in the park you did not see the park to your satisfaction, you could purchase an additional day. The tickets worked like Magic your Way base tickets, in that they could only be used at 1 park per day, so if you bought a 2 or 3 day bounce back, you could visit 1 park per day for each day, and you had a limited time before bounce back days expired.
I beleive these are now obsolete as you can upgrade a MYW base ticket after purchase for up to 7 days.