A Coma Slide is primarily used when offense attacks the goal with a drive from X.
Let's say the ball is behind the goal at X.The attackmen at X will drive it up one side or the other.
The on-ball defender's responsibility is to not get beat top-side. This means the defender needs to position his feet and body to force the ball carrier to roll back towards the endline and behind the goal, preferably before the ball gets above Goal Line Extended (GLE).
Good attackmen get used to this and will drive into the defender in an attempt to get above GLE and to a place where they can be a threat to score.