GMC 2010 Yukon/Yukon XL Owner's Manual page 15

If equipped, this control is located on the outboard side
of the seat.
On vehicles with two-way lumbar, press and hold
the top of the control to increase lumbar support.
To decrease, press and hold the bottom of the control.
On vehicles with four-way lumbar, press and hold
the front of the control to increase lumbar support.
To decrease, press and hold the rear of the control.
To raise the height of the support, press and hold the
top of the control. To lower, press and hold the bottom
of the control.
See Power Lumbar on page 2 7.
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