If during Execute phase you hit 'Execute' without a Sudden Death proc up it does dump all of your rage. (Execute functions as normal)
If the proc is up when in Execute phase - It treats the ability as per the normal rules of Sudden Death that Rallik outlined above.
Ok, had it implemented so that SD below 20% was just the '10 rage left', but I'll change it, thanks.
r2125 has SD now implemented as you stated