QF Safety video, says do this, does opposite

[JohnK]

Unless you're stretching the limits of the seatbelt (as many probably are nowadays) I can't even see why you would unbuckle. Maybe loosen a little, but it's virtually un-noticeable so what's the payoff?

Loosen a little? It's already stretched to the limit.

 

[Vic]

That's why you need to lean forward, the forces of sliding push you backwards...