Death Knight - 120% extra pet damage

[smusen]

If you equip 2p of T17 DK set (Unholy) and cast dark transformation and quickly equip the current T18 set, you'll gain the benefit from both sets.
Baseline dark transformation gives 200% pet damage, which is why the video is showing 320% plus the 10% damage and 15% attackspeed to both you and the pet.

You can macro the gear swap and dark transformation like this

/equipset *Name of set profile*
/cast Dark Transformation

[frozenspell]

Nice find!