I not quite sure how this proc effect is suppose to function. I tested it on various mobs with differing results. On random mobs the effect would throw the NPC into the air, take 100 damage, and stun them. However, the stun seemed to last about a second or two, not the 7.5 seconds stated. Also, while the "whirlwind" debuff was in effect the NPC would move towards me while still levitated in the air. I was under the assumption that the effect would levitate them and stun them for 7.5 seconds. Afterwards they would fall to the ground and continue attacking.
On Plane of Hate trash the Mob was thrown into the air for only a second before immediately coming back down and was stunned for a second like other melee stun procs. So in this case neither the stun or the levitate effect lasted more than a second and multiple mobs.
On boss mobs they would only take 60 damage, not 100 damage due to MotM buff, and were immune to the stun effect and levitation of this proc.
So, I am not quite sure how this weapon's proc is suppose to function, because none of the NPCs i tried it on were stunned for the 7.5 seconds stated and even managed to move towards and attack me while still being levitated and under this whirlwind effect.