Leave it to Chad to bring up the one throw in the whole syllabus that I understand least. We'd just finished a miraculous workout in which we investigated the otoshi and guruma throws in judo. I'd assigned him some open-ended homework of looking at all the things in the judo syllabus that are named either otoshi or guruma and figuring out why they are named as they are. He chimes in, "What about kataguruma?"
Well, first off, kataguruma is NOT my throw. I can barely get the iron cross variant from nagenokata to work on someone half my size. I have a variant that I can throw to some effect occasionally in randori, but it is not really a guruma. It is more of a home-grown taiotoshi with a shoulder in the mix as a fulcrum.
Guruma throws tend to make a very distinctive fall for uke as he rotates in two planes - head-over-heels, and around a vertical axis through his body. It very much resembles a cylinder rolling on its axis as it turns end over end. Kataguruma has this fall when thrown properly.
Part of why kataguruma is so confusing an example of otoshi vs. guruma is because it is typically taught with a parallel offbalance down the line of uke's feet. This offbalance is typically associated with otoshi throws. In this case. tori offbalances as if to throw an otoshi and as tori compensates with the rear foot he gets lifted in a guruma throw.
Last night I had a sudden thought regarding this. Often, the otoshi-guruma thing makes more sense to me when I can find the otoshi variant of the guruma that I'm studying or vice versa. In this case, what is an otoshi that is thrown from nearly the same position and timing as kataguruma? I think it is tsurikomigoshi. I'll need some time and skilled ukes to play this idea, because kataguruma and tsurikomigoshi both intimidate most ukes, but I think that playing for a while with these two throws in a compare and contrast type practice will help to clarify why kataguruma is a guruma.