In reply to Rory Shaw:
They shouldn't!
They go round the rubber rand and there is a strip that runs under the foot just in front of the heel which shouldn't get walked on very much.
However, in some boots (left one in pic) you can see it's not a straight line and the left hand side of the left pic there's a bit of the heel section poking forwards that nearly clashes with it. The right hand boot is easily compatible, the left is not, the rubber gaiter will get walked on and damaged. If it's eBay purchase you should be able to resell it if not compatible.
The other issue is crampons given it sits on the rand. If you have boots with a toe bail groove you *should* be able to seat the yeti just above the groove. Same with heel lug. With strap on crampons (or no crampons) it shouldn't be a consideration.
Depending on the boot you can get a very slight gap under the boot on the inside edge so you can't use them as waders and expect deep water waterproofing (some people have perfect fit and super glue and can achieve a full seal) but certainly snow, slush, wet grass, some bogginess they are a game changer. Slight warmth increase too but I don't want to overstate that as it's slight, unless you have specialist insulated ones (rare)
Post edited at 12:13