Backspin: Swing Speed or Quality of strike?

Quality of strike of course.

My pro has a good explaination for it.

If you hit it thin, ie on groove 1 or 2 of an iron it will go low and have little back spin. If you hit it on grooves 3 -5 it will be out the sweet spot and rise up more, with groves 4 and 5 giving the better shots as you get closer to the sweet spot which is around grove 4 or 5 on an iron.

That is why if you hit a shot a little low on the face it can still go miles but without the backspin to give it lift it does not drop down soft and under control.

The same rule applies from everything from a driver down to a 10 yard chip and everything in-between.