GMC 2011 Sierra Owner's Manual page 364

Minor brake pedal pulsation or
pedal movement during this time
is normal and the driver should
continue to apply the brake pedal
as the driving situation dictates
The Brake Assist feature will
automatically disengage when
the brake pedal is released or
brake pedal pressure is quickly
decreased.
2500 and 3500 series vehicles with
StabiliTrak have a Hill Start Assist
(HSA) feature, which may be useful
when the vehicle is stopped on a
grade. This feature is designed to
prevent the vehicle from rolling,
either forward or rearward, during
vehicle drive off. After the driver
completely stops and holds the
vehicle in a complete standstill on
a grade, HSA will be automatically
activated. During the transition
period between when the driver
releases the brake pedal and starts
to accelerate to drive off on a grade,
HSA holds the braking pressure
for a maximum of two seconds
to ensure that there is no rolling.
The brakes will automatically
release when the accelerator pedal
is applied within the two second
window. If the vehicle is equipped
with the Integrated Trailer Brake
Control (ITBC) system, HSA may
also apply the trailer brakes. It will
not activate if the vehicle is in a
drive gear and facing downhill or
if the vehicle is facing uphill and
in R (Reverse). There may be
situations on minor hills (less than
5% grade) with a loaded vehicle or
while pulling a trailer where HSA
will not activate.
The vehicle may have a vehicle
stability enhancement system called
®
StabiliTrak . It is an advanced
computer-controlled system that
assists the driver with directional
control of the vehicle in difficult
driving conditions.
StabiliTrak activates when the
computer senses a discrepancy
between the intended path and
the direction the vehicle is actually
traveling. StabiliTrak selectively
applies braking pressure at any one
of the vehicle's brakes to assist the
driver with keeping the vehicle on
the intended path.