When my guineapig started popcorning the first time I thought there was something seriously wrong with him! It's such a mad thing to do but I love seeing them do it - after all it shows they're happy.
The first time you see it, you'll think they're having some sort of seizure. But I can assure you, they are extremely happy! It is a sort of jumping/twisting motion, usually accompanied by running and happy wheeking.
I was going to say the same thing Mn! Except when my CoCo popcorns, she doesn't twist, she comes straight up off the floor about an inch with all 4 paws.