A support or load leg prevents the downward rotation of the seat. Crash test videos:
Video Link: http://www.youtube.com/watch?v=T_Cht2vkxZw -- note how the seat first moves forward and downward, becoming almost horizontal,
Video Link: http://www.youtube.com/watch?v=_x3KaPwZBXI -- the seat stays much more upright [the seat is Euro Aton 2 with an isofix base].
After the seat has moved forward/downward, it moves back up towards the seatback -- that's the rebound. Anti rebound system might be an anti-rebound bar on the seat's base, seat's handle requiring "up" position in car, bolsters on the seat where it's pressing against the seatback, or tethers [not on infant seats]. It's mean to lessen the upward movement so that the child seat doesn't slam into the seatback.
[Added] Ugh, why does it automatically embed videos into the post? :/