GMC Yukon Denalid Owner's Manual page 28

The seatback will fold forward. Leaving the
seatback in this position creates 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.
28
3. Lift the lever again
to release the rear
of the seat from
the floor. The seat
will tumble forward.