Journeys through time are always a tricky subject. First, the topic has been used and reused in sci-fi and fantasy stories for the umpteenth time, and not always for the better. Second, the scientific approach to what may be possible and what not is not unanimous.

In my story, the prime directive is: We are dealing with a reality in which the abstract phenomenon of death is walking around as a person and in which objects that have no technical validation whatsoever like a sand-filled hourglass can be used to create gaps within the time-space-continuum. I would therefore advise the ecucated physicists among my readers to NOT start thinking in scientific terms when it comes to my story. Unless you can give me sound proof of the non-existence of Death or the improbability of the special functions of the hour-glass, I won’t accept any objections.

I would rather rely on common sense in this matter. The Beetle and its passengers are part of a singular system. By hitting the brake, Mercedes terminated one aspect, the spatial one, of the journey. She had of course no influence on the temporal aspect, because she didn’t hold the hour-glass. But being part of said system, her action did have an effect.
Another question could be if the passengers of the system could really leave the car while within the temporal flux-state. When I wrote the story, I was loosely inspired by the TV show “Star Trek: Deep Space Nine”. In the first episode, a Runabout with two passengers accidentally ends up and gets stuck in a stable worm-hole, where it comes to a halt and the two persons leave the vessel. I liked the idea of an area with determined space but undetermined time, which is where Bonnie and Mercedes are and which also explains why they don’t see any surroundings, because they are not temporally defined.

About the grains of sand:
Since the sand, like the rest of the system, is within a flux of time, they actually should ALL be in an undefined state like the sand in this picture. They are, but the story would be rather dull to read if you only had specks of colours instead of characters to look at. Or on a more scientific level: It is actually they, who are undefined, whereas reality is of course very stable and time is not separated from time. We only see it from their perspective. As for the sand: It works as some sort of indicator, a detector of what’s happening. Like the clock in a common time-machine tends to run in fast-forward or rewind-mode, to visualize what’s going on outside, while the time within the capsule is still moving in normal speed on its linear way. The sand therefore shows that the temporal system is in an unbalanced state.