I found two videos where the PF quickly moves the speedbrake lever during rollout on the 757. Note that they are already deployed. Here are the links: video 1 and video 2 (out of view). What is the purpose of doing so?
On B757, B767, B777, retracting the speed brakes during the landing roll is a technique some pilots use to disarm the Auto Brakes. I haven’t flown the other Boeings, but they probably also work this way.
It is especially useful if your Auto-braking is coming on a little too strong and you want to extend your rollout to the next exit.
It is easier than reaching all the way over to the right side of the panel to turn them off.
Pushing hard on the brake pedals will also disarm the Autobrakes, but that is the last thing you want to do when trying to extend your rollout.