Backtrack is a popular feature in cheats. It's an exploit that makes delayed aim act as proper aim.
It works by logging the players' hitbox positions for the X previous ticks, and then abuses the game's lag compensation in order to make the server think that the player is hitting someone even though in reality, their aim is incorrect and delayed.
This plugin works by not allowing players to decrement their usercmd->tick_count.
Examples of backtracking in...