Pretty straightforward, but requires Dual Spec -
1. Buff someone in your party/raid with Focus Magic as Arcane
2. Switch to your other spec
3. The Focus Magic buff doesn't go away on the buffed player and you still get procs from it despite not even being Arcane
4. Switch back to refresh the buff as necessary
5. ???
6. Profit!
Useful for competitive mages who want to increase their own DPS and raid utility