The goal of electronic trainline brake activation is to have the entire train set up almost simultaneously to avoid slack and make the train more quickly respond.

When you initiate a brake pipe reduction on the current system, the cars set up from the head end (the first to see the train line reduced), and slack will run in, although somewhat gently (as one car at a time sets up). If you want to stay stretched you use enough power to do so.

If you are using, or preparing to use dynamic braking, you want to gently gather up the slack with the independent or dynamics, to bunch the train.

I've only used the GE train simulator a few times at locomotives demonstrations since I left engine service (to show off for my staff), so I am unfamiliar with most of them. How they program the effects of slack, or the effects of terrain and gradient on slack are a mystery to me.