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 responsiblilty 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. The most desirable place for the driver is to get to the spot 5 steps to the side of the goal and 5 steps above GLE, which is called the Island, where they have the full options of shooting or driving to shoot.
If the defenseman is doing a good job of staying topside, the ball carrier will not be able to run around or roll around outside over the top of the defender to make a move to the center of the field for a shot. Instead, the ball carrier will be forced to roll under using what's called an inside roll to make that move to the center for a shooting chance.
Within a Coma Slide package, while this is happening the crease defender needs to lock down his man. This will prevent an easy dump pass in front of the crease. The d-middies up top need to have their heads on a swivel and be sluffed towards GLE ever-so-slightly, ready for the 2nd (backup) slide.
Then there is the issue of the two other defenders -- depending on which direction the offense is driving from X, the defender who is guarding the attack on the wing in the direction of which the X Attack is driving needs to lock off his man (this prevents a dump pass and a reset of the offense). The pole on the other wing needs to start to gradually sluff in towards the crease, to the point where when the driving attacker is turned and starts his inside roll the remaining defenseman can slide across the crease. This slide is the Coma Slide, for "COMe Across the crease".
When the slide goes, just as a safeguard the middies up top need to sluff in and split the attack on the wing and their o-mids up top.
Below is a diagram of the Coma Slide out of 3 different sets.