The theory behind 'Fenestron Stall' was that even in the hover, the vertical and heavily cambered area of the tail was producing useful anti-torque lift by virtue of the fact the air was accelerated over the fin on its way to the fenestron. Therefore the fenestron itself wasn't having to work as hard it might if that lift wasn't there. Now introduce a wind from the right (about 10 kts was the quoted figure) that disrupts that flow around the fin and suddenly the fenestron is behind the drag curve, not producing enough thrust to make up for the loss of the fin lift - the yaw to the left starts and what is required is an unusually large right pedal input to cancel the yaw and compensate for the loss of lift from the fin. Most pilots would be reluctant to make such a large yaw input initially and so the yaw rate increases - all the while the selected pitch of the fenestron is less than is actually required because the pilot is having to play catch-up.
The theory was debunked by Aerospatiale because they showed that full right pedal would always stop the yaw, even if it overtorqued the transmission!
Frankly I am surprised the French didn't claim it was designed in like the jackstall due to underpowered main jacks
