That's not so easy, it depends on the train type. This is shown by the indicator lights (eg the one with 85 in it). When active, your watch speed is 85 Kmh. That means, if you are driving with PZB, you will get a signal, which is For example, if you expect to stop, you must confirm, and you will have to slow down to below 85 km / h within a certain time, otherwise you will get an automatic emergency brake.
Good information with grafics (but in German) you will find here
http://www.tf-ausbildung.de/BahnInfo/pzb90.htm
But good PZB trains are only ChrisTrains or Virtual Railroads Expert Lines.