The wording is in fact ambiguous. An easy way to clarify the rules is to append the intended meaning of it.
Another thing: Today I deliberately tried the banned stance. I have only a normal sized bag but I rolled it tight, put it halfway under my ankle and the rest of it protruded enough between my legs to form a solid rest for my buttock. It was very comfortable, but thats it - nothing more. The wobble in my aim still came from my left knee where the rifle balanced on my hand - nothing changed, no better score. As long as there is no support for the left knee, it makes no difference. So why prohibit it? OK, it was already banned before, but why bother and annoy all us ankle handicapped shooters with new and overly complicated regulations?