From the steam forums
Wilsonam
Roughly, ZEDtime kicks in dependent on cool shots - long-range headshot, messy zomb death etc. Algorithms in place to stop it happening too often.
And it actually helps - in much the same way as the old Max Payne "Bullet Time". While the zeds and everyone else is slowed down, the players learn how to use it. You can get a couple of well-aimed shots off quickly enough to really use them to headshot a couple more.
So not only do you all get to watch slow-mo gore, you also get to pick off a couple more in the second or so you have
Ramm-Jaeger
Its not really something you have, its more like an event. You don't get to choose when it happens, the game chooses when it thinks you've done something cool. Its random, and only happens occasionally and for a short time. Think of it as a reward for pulling off something cool. Everyone does slow down, but it is very short and doesn't really bother you.