GMC Yukon XL Denali Owner's Manual page 64

3-10 Seats and Restraints
2. Lift the lever, on the outboard
side of the seat, to release the
seatback.
The seatback will fold forward to
create a flat load floor.
If the seatback cannot fold flat,
try moving the front seat forward
and/or put the front seatback in
the upright position.
3. Lift the lever again to release the
rear of the seat from the floor.
The seat will tumble forward.