I think the understays are for at-home or when you're outside doing vigorous activities, so your stays would just be laced tight enough to support. When you're ice skating or whatever, you don't lace down completely or you wear an athletic corset or whatever.

The slippers are really cute, and are one of those vintage patterns that you still see variations of more than a hundred years later. Knitty had a pair with a Mary Jane strap a couple issues back that looked pretty much identical, minus the ribbing at the ankle. I think these would be hilarious with some sort of extremely large pompom at the heel.