Well, in layman's terms, the higher it is, the shorter the delay between individual lemmings exiting the trapdoors. In terms of a mathematical equation, I confess I'm not entirely sure. I'm not sure how much of a help this is, but:
- Lemmings come out about twice as fast at 50 than they do at 01.
- Lemmings come out about twice as fast at 80 than they do at 50.
- Lemmings come out about twice as fast at 99 than they do at 88.
I haven't tried it in other versions, but on a custom level in WinLemm once, I accidentally set the release rate to about 250 or so, which caused the lemmings to come out extremely bunched up. I think there was only a single frame's delay between each lemming, so at an educated guess, I think it is related to how many frames pass between each lemming, something in the form of a(b - x), where a and b are constants (a might be simply 1) and x is the release rate. And if b - x is zero or negative, then it naturally pegs at 1 frame due to the way it is programmed.
I may be way off, of course, but that's just from a programmer's and mathematician's observation.