Ok, well I dont know about the 757, but I do know about the 767, and I imagine that they will be fairly similar.
Given my recent sucess on this forum I'll be wrong anyway, hehe.
Ok,
Activation of runway align occurs at 500ft RA, activation is not displayed, autopilot will initiate a slip with maximum bank angle of 2 degrees when crab angle exceeds 5 degrees, wing leveling from the slip is initiated when rollout mode is engaged.
The purpose of this is to reduce the crab angle prior to an automatic landing.
Ok, so what does that mean?
At 500ft the autopilot will put in a bit of bank to help compensate for a crab angle due to crosswind. This bank is held until rollout is activated at 5ft RA. The idea being the aircraft will land wings level, but with a lesser crab angle, the theory being that the aircraft will not drift from centreline during that last 5ft.
It is then the responsibility of the AUTOPILOT to control the rudder and nosewheel steering to maintain (or regain if required) the centreline until the aircraft is brought to a complete stop, or the autopilot is disconnected.