There are two instances happening at the same time.
Cutters from the back of each stack cut deep to start.
The thrower fakes and the cutter comes under for the throw.
While the disc is in the air, the cutter from the back of the opposite line cuts deep.
When the first cutter gets the disc, they fake for the deep cutter to come under for the throw.
The original cutters clear to the opposite color line and a new deep cutter starts cutting deep
The drill continues - working specifically on timing cuts for the under