Sorry I'm late to the party here...if you are still looking for a solution, there are a few ways to cut out pieces of a loop.
The ones you found are functions named TrimStart and TrimEnd. As Alex says, these are "instant" functions, they have the immediate effect of creating a new loop layer by lopping off either the front or end of the loop from wherever you are when you press the button.
I think you want something different though. When you say " I cut a smaller piece which can start anytime and which can end anytime", this is something that goes by the non-obvious EDP name "unrounded multiply" and how you use that can be puzzling if you're not familiar with the EDP.
For the most basic use, you start your "cut" by first using the Multiply function at the start, then let the loop play, you will be in Multiply Mode. When you reach the point where you want to end the cut, press Record. This will then splice out a section of the loop between those two points and it immediately starts playing.
The reason this is bound up with the Multiply function has to do when you let this end point carry over across the loop boundary and overlap with the start point. Once that happens, the loop will replicate (or multiply) itself and keep going. You can cut it off at any time, but the resulting loop will be longer than the original loop because you let it repeat once before you stopped the multiply mode. It's kind of hard to describe with words, just try it out. The key is that you will need both the Multiply function and the Record function bound to a switch. Everyone has Record but not everyone has Multiply unless they know about this.