What does the hourglass option under Duration do?

In the Time Slice Editor, there are two buttons with hourglasses in line with the Duration fields. There are no tooltips for these options and I’m not finding them in the documentation (potentially because I don’t know what they’re officially called, but still).
What do they do?
2023-01-03 16_09_57-Create New Time Slice

They are toggle buttons; either one or the other is pressed. When the one on the left is pressed, changes to the duration will shift the end time of the time slice. When the one on the right is pressed, changes to the duration will shift the start time of the time slice. Since changing the duration has to change either the start or the end, these buttons let you decide which of them is “anchored” and which of them is adjusted.