Took me a while to understand how this works, till I read @MusicalBox’s reply, which I’d rephrase as shrinking and stretching a clip, i.e. changing its speed, instead of trimming, to make it shorter or longer. I’d more intuitively associate that with Alt than Ctrl, but if Ctrl is what other packages use, that’s probably what shotcut should stick to.
UPDATE: I just saw @Micha asked and suggested precisely that here, and a solution was given using a (Time Remap) filter. I trust it works, but it’s far from being as intuitive as CTRL-drag.