@René-Zuber Simply click on that "Split Route" menu entry and a new window pops up with a list of all the waypoints.
Clicking a waypoint marks it as the start of the part of your route you wish to split into a new smaller route.
I.e:
Clicking 3 waypoints, will create 4 new routes. One from start to waypoint click 1, one from waypoint click 1 to waypoint click 2, one from waypoint click 2 to waypoint click 3 and one from waypoint click 3 to the end.
Note how every way point "marked" by clicking it, is indented in the list.
Hope that makes sense.
See example below. Every waypoint marked with the arrow is a split point. In this example I've split the route into 4 parts by clicking 3 waypoints, which are indented:
After clicking Next you can name the 4 newly created routes: