Up-Smash with control stick performs the following checks in order:
1) Instant A press
2) Control stick Y axis > 0.66
3) Control stick Y axis cardinal change within last 4 frames
Cardinal change occurs when stick is moved past 0.25.
(It could be "greater than or equal to" 0.66 but I didn't look too intently.)
Many actions requiring stick inputs follow this "meet threshold within frame window" logic.
https://smashboards.com/threads/mel...-here-in-the-op.247119/page-116#post-18806802
In regards to that shield dropping post, notice the frame window difference between spot dodging and shield dropping. Because the timing for spot dodging is tighter, you can do something as follows to buffer a shield drop:
Land normally on a platform. On the first frame of landing lag, hold L and -1.00 control stick y-axis (all the way down). Even though your stick is all the way down (which you normally associate with spotdodging), you will shield drop because the 4 frame window check for spot dodging fails.