A muzzleloader sabot relies on the lower skirt to expand into the grooves to impart spin.
I think that the wall of most every sabot fit tight enough in the bore that they do not need to rely on the expansion of the base to begin spinning. Thus they would begin to spin immediately upon movement.
The walls of the sabot also slightly bulge out when the bullet is simply inserted into it, and this helps sabots to fill the barrel grooves and to spin even as it is rammed down into the breech without the skirt being expanded.
There are some looser fitting sabots and I don't know if it takes any longer for them to begin spinning in some bores or not, but they are said to load and shoot better out of some guns with tighter bores.
Also, the TC Break-O-Way sabots don't have a hollow base skirt but a flat wool wad base which I don't believe is even required for them to begin spinning since the sabot wall is plenty tight. The skirt may help to create more consisent and higher velocity and maybe even more RPM's in flight, but is not really necessary to impart an earlier spin.