GMC 2010 Yukon XL Denali Owner's Manual page 43

The seatback will fold forward automatically.
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.
3. Lift the same lever
again to release the
rear of the seat from
the floor. The seat will
tumble forward.
2-11