He quite literally leaves his feet. It’s 100% unavoidable by not delivering the hit.
You must be from the 60’s NHL if that’s a clean hit to you.
"Unavoidable by not delivering the hit" is... I'm trying to figure out a way to say this diplomatically, but I'm afraid it's just ignorant of what the rule book is setting out to explain in Rule 48. The word unavoidable is used purely in the context of an understanding that the puck carrier is legally available to be hit, and that the hitter is fully allowed to make the hit despite the risk of head contact.
All hits are avoidable by not making them. That is a redundant observation. So when the book makes allowances for hits to the head in which head contact is unavoidable, they obviously don't mean "the player had no choice but to hit him."
What this comes down to is not whether head contact was unavoidable, which it may have been, but whether Pospisil did anything to accentuate or increase the head contact. I believe it will be found that he did, both through the motion of his body upwards during the hit (whether he actually left his feet prior to the hit or during the contact really doesn't matter given there isn't a reason for him to elevate from his initial position other than to do what he did) and the extension of the elbow afterwards. And given this comes on the heels of another suspension, this should be an in-person hearing.