GMC 2010 Yukon XL Denali Owner's Manual page 50

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.
2-18
3. Lift the lever again
to release the rear of
the seat from the floor.
The seat will tumble
forward.