I was in a H model Huey, in a 4000+ft/min updraft (I dont know exacty how much it was as the VSI only goes to 4).... when 1. it suddenly stopped or 2. we hit a downdraft.
We were well less than 0 G, as witnessed by the bags of cargo that hit the roof then 'floated' forward.
The acft pitched to 50+ degrees nose down and rotated rapidly to the right. (From memory it wasnt a violent or bumpy rotation, like 'jack stall' or RBS)
Needless to say, flew home to change my shorts!