Everytime I need to loop a portion or a specific segment from a video, musical track or even a part from a sequence with images I forgot how I was doing it. So right now I will write in a few steps how it’s done.
I am assuming the following scenario: I have certain amount of videos/images or different kind of source media files – in this case 3 videos.
And I want to loop only a specific part of this sequence.

The first step is to mark the desirable portion with the Mark In and Mark Out buttons or the default short key for that operation is I for Input – starting point and O for Output – the end point.

The second step is to select the Loop Icon.
If you press play right now then the whole sequence will start to loop from the beginning of the timeline. Right now this is not the case. So we select only the loop icon (not hitting play) and continue with step 3.
The default short key for that operation is: Ctrl + /

The third step is to select from the falling main menu -> Playback -> Play Around/to -> Play In to Out
The default short key for that operation is : Alt+/

The looping cycle will start right after choosing that option.
It’s good to mention that if you click deliberately or not on the red slide bar then the looping cycle will break and you can left with the impression that this operation is not working. But if you click again the combination for playing back that segment: alt + / the loop will restart from the beginning of the in point. If you mouse click outside the marked segment then the loop will still continue to play. So the main concern is to be aware if you are clicking into the marked area or outside of it.
Voila! That’s it.