According the the guide, Fel Rush should be used above Blade Dance and Chaos Strike when it is about to cap and should be used first on the pull.
Icy Veins:
1.1. Opening Rotation
Use Potion of the Old War Icon Potion of the Old War just before the pull.
Cast Nemesis Icon Nemesis.
Cast Felblade Icon Felblade to engage.
Cast Fel Rush Icon Fel Rush.
Cast Metamorphosis Icon Metamorphosis.
Cast Chaos Blades Icon Chaos Blades if taken.
Cast Fel Barrage Icon Fel Barrage if taken.
Cast Fury of the Illidari Icon Fury of the Illidari.
Cast Death Sweep Icon Death Sweep with First Blood Icon First Blood.
Cast Annihilation Icon Annihilation.
Continue with Single Target Rotation.
What is the reasoning in using Fel Rush before capping at all? Like why not just leave it out and spend your time on higher damaging abilities?