Excute causes a global cooldown. HS can be used even if the GCD is cooling down.
But I have to agree. Execute should have the highest priority after CS. It even has a higher priority than BT, because the rage is genererated with damage: If you don't have enough rage, then use BT, but if you have enough for execute, then EXECUTE.
The whole rage generation thing doesn't really count in that short phase where the boss is under 21%. In that phase only two things are important:
1. Is CS up? NO => Use it if you can. YES => EXECUTE
2. EXECUTE
BT should be used only if there is not enough rage for execute. Not primary to have enough rage for execute.
I know, what nomnom wanted to do. He wanted the routine to do as many executes within the uptime of CS. Therefore BT has an higher priority than execute. But that is absolutely wrong. And the use of RB or other abilities (except of HS) is the biggest mistake during the <21% phase...
I hope my bad (drunken) english made some sense.