Engaging the INCR speeds switch will cause the stick shaker/pusher to operate at a relatively lower AOA than the normal preset value.
One small point - my understanding, based on the testimony at the NTSB public hearing, (lots of good background info in the video archive from day 1) is that the selecting the REF SPEEDS switch to INCR only affects the stick shaker. It does not affect the stick pusher. The stick pusher, by design, even with no ice on the wing, fires after the aerodynamic stall anyway. It is not there to protect against the stall, but to provide stall identification under some conditions when the aircraft nose does not fall at the stall. It also is intended to ensure the angle of attack does not continue to increase to high values. All assuming that the pilot allows the stick pusher to do its job.