I'm no distance slinger, but I can definitely agree with his concept of the "resonance" of any given sling. It's something you can feel swapping between different slings even just with tennis balls, assuming you slow down enough to really feel the sling as you throw.

Sling weight, weight distribution, stiffness, projectile weight, projectile density all have an effect on this, so you have to find the perfect balance/speed for your combination of factors to get the most of out your throw.

I think this is the same effect I've seen with slingers that normally use heavy/stiff slings and then test out a light/floppy sling. They try to "muscle" the throw like they would with their heavy sling, but you simply can't put energy into a light sling in a short period of time like that - it can't "keep up" with the acceleration in a short period of time and you end up with a fouled shot. You have to spread the energy over the whole powerstroke rather than trying to force it in within the last 30% of the stroke.