When a character with Backlash wins a round the ability gets activated and affects you (the owner of the backlash character).
It's usually prejudicial for the player. See Beef, for example:
Backlash: - 2 Life Min 2
If Beef wins the round, the number of Life points of the player controlling Beef will be reduced by 2, or up to a minimum of 2. If the number of Life points of the player controlling Beef is lower than or equal to 2, then the ability will have no effect.
Put it this way: say you're using Beef.you win the round with him.he does his damage normally,BUT he inflicts two damage to you(at a minimum of two)..then there's Gork,who does the same thing with poisoning yourself,and Norman,the card who reduces your pills.