The Split method does not consider the selection. It just operates on
the event that you call it on and the Timecode you pass in is relative
to the event start.

Another tricky thing is that splitting increases the number of events
so if you split in the middle of an enumeration, the counter may get
thrown off. You may need to enumerate the events first without
splitting. Just collect each "target" event in your first pass then
enumerate your target collection in a second pass to do the splits.