I'll simplify it.
Round rolls across another object better than angled(ie: any form of a corner which applies extra friction or pressure), and that is relevant in ALL possible forms of play.
Take the ends of a rope in both hands and rub it back and forth on the corners of an open doorway. Now imagine doing the same thing but with the corners rounded... there is less friction resulting in faster, easier movement.
Try rolling a square block...no explaination necessary.
Try sliding down a flight of stairs on your back. Then do it again with the edges of the stairs rounded off, you WILL go down faster.
square tops as opposed to rounded tops:
Imagine you're doing an upwards figure eight, this is what's happening as the cord has to make contact hitting the corner at the top, making a "shhhk" noise as it slides past.

It isn't an amazing amount of difference, but it's still a difference. It's like when racers do whatever they can to make their car go faster/ect.
This is the exact reason why BB chucks seem/do flow smoother and go faster than cord. There is NO grind. If you look at your BB chucks, the first link of the chain extends out past the top of the chuck like an arm, making it so the chain doesn't rub the top of the chucks during use. See where I'm going with this?