Maybe ok for the timing, but regardless of your momentum if you keep on holding to a player who lands head first you are causing a dangerous tackle. You can't argue that "it's not his fault, it's momentum" and that "it was a proper tackle" simultaneously, and yet not take dangerous play into consideration. »
When I barge into a ruck, I generally look at where my studs land. I don't think any referee would allow me to walk all over someone's face and argue for my defence that my eyes were on the ball the whole time.