GMC Yukon Owner's Manual page 55

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.
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