I do not use visual cues or sound at all. For me, its been this intuitive feeling regarding when to press the button.
Its a really similar feeling to when you try buffering a dash out of a dthrow or perhaps when changing up your laser heights from a short hop. Anytime you have to delay your inputs to properly time something; its that sort of intuition.
Usually I will try SideB a few times to calibrate my timing to get use to shortening. Say I get the shortest shorten a couple times, I'll try waiting a wee bit longer. I usually do not have the accuracy to get the exact shorten I would like (its a one frame window >_>), but I can usually get a shorten of any length (3 frame window).
I originally tried doing a visual or sound cue, but I found it changed too much depending upon my reaction to it. Plus, I would be waiting for the visual cue and sort of expecting a count down when practicing. In an actual match, that sort of situation never happens. Its like a spur of the moment after gauging my opponent that I decide to shorten.
So, I'd say to skip trying to get use to a visual/sound cue and build up more of a muscle memory for shortening. For example, practice specific waiting periods between sideB and pressing B again. If you keep doing that, you should eventually build up a muscle memory for shortening and find yourself getting it far more frequently. No idea how to improve which shorten you get other than keep fine tweaking your delays.
