Provoke + Shirk is what let's Tanks stay in DPS stance without ever losing hate to a DPS.

Main Tank: 10,000 Aggro
Off Tank: 4,000 Aggro

Off Tank uses Provoke:
Main Tank: 10,000 Aggro
Off Tank: 10,001 Aggro

Off Tank uses Shirk on the Main Tank:
Main Tank: 12,500 Aggro
Off Tank: 7,501 Aggro

Repeat over and over and the DPS are never able to catch up. It also makes Tank swaps instantaneous without ever having to use a Enmity combo.