I'm not sure... There was a lady standing at her kitchen window and saw me going around the curve, come off the bike, and saw the helmet come off before I hit the ground.
It was a full-face that could flip the chin up, but it didn't have a positive locking mechanism (I.E. PLASTIC HOOKS instead of metal). The cop found it in the woods and brought it to me, and I remember the face being flipped up, and how that was odd because I had been riding with it locked down. The only thing I can imagine is the wind caught under the chin, flipped the face up, and pulled it off my head backwards.
We tested this later, and the helmet could indeed could come off with the face flipped up and the chin strap tight (we didn't even have to wrestle with it). Don't ever buy a VCan modular helmet... You might get your head knocked off!