The whole Sand falling is obtusely the shield getting closer to perform the magical ability to go back in time, after all Homura's soul gem at the start of a loop is perfectly clean, despite having done a magical wonder greater then everything short of Madoka's rise to godhood after all, Homura's perfectly clean soul gem after that magical act must be some complex shield magic thingy.
As for examples to signs that my idea of how Homura's time stop work well, when Homura is scaring Kyoko Homura's timestops are instance their is no warning no lag no 'hearing the clocks on the shield make noise' Vs the time in Rebellion were Homura's Timestop is so slow to start that Sayaka can interrupt it and stop it from happening. My reasoning thus points out that Homura's shield is slow due to the fact that is was used a lot recently in big ways. Another example of when the Shield is working slower then Homura's scaring of Kyoko is in the anime after Kyoko got the jump on her, the activation of timestop was slow enough that she hear the click of the shield before the flash bomb went off.
As for a amusing point to this whole thing, Word of god means little when it is canon that a Magical girl's feelings can effect her own magic, if for instant Homura believes the same theory I do, then her Magic could very well work how I think it works, and considering Homura is aware that her shield can stop working but hasn't noticed a time limit on her timestop (aka she knows it can stop working but seems to not know it when it will happen) now unless you think Homura hasn't come to the conclusion that she cant stop time when her shield has fully spun the hourglass in it, and considering the fact that she wasn't prepared for it to stop during her battle with Wally, is it possible that she is able to stop time when her hourglass if full cause if she can then I am right and if not then I am wrong.
Now if you want this to keep this going we should move it over to Mami's house.