Can someone please explain what this 72 IAS breakpoint is, and what the difference is to the 42 IAS breakpoint?

42 IAS = 9 frame attack
72 IAS = 9 frame attack .....yet it's supposedly faster? (smoother?) in some way?

72 IAS is on maxroll, DarkHumility and others talk about it, but I'm yet to see anyone explain it well.

I assume it's not really worth it, but still curious what the actual difference is.

Thanks