The middle piece doesn't rotate like a roller, but the two sides of the bit swivel independently similar to a Myler.
Big Idea Eventing
I've been told mixed things. I had a R judge tell me yes (he went on to show one of his own horses in one) and had folks on here tell me no. My gelding chewed his up & I didn't replace it.
Proud supporter of SprotHorseRiders.com
It is my understanding that it is one of the bits that is covered by
EV Appendix 4 Permitted snaffles
15. Ordinary snaffle with rotating mouth piece
and the accompanying diagram
To be sure, check with USEF (not USEA)
chief feeder and mucker for Music, Spy, Belle and Tiara. Someone else is now feeding and mucking for Chief and Brain (both foxhunting now).