I would not what to call it but it's very close to going to an inverted flat spin. Just a little more down elevator travel and it probably would have flatened out and continued to spin but slower. Throttle may do the same thing if there us enough power.

I do not normally answer a question with a question:

What plane is it?

Was it rolling nose low?

Did it seem as though it was rotating around a wingtip? or were the wings staying "level" as in the tips pointing to the ground and it was rotating around the center of the fuse.

My LT-40, would do that in just the same configuration, Nose straight down, wing leading edges pointing to the ground and rotating like mad around the center axis of the fuse. Thought the wings were going to come off but it sure looked cool and I could reproduce it at will. My Something Exta did it also until I increased the elevator throws then it turned into a beautiful flat sping