SLR stands for SHIMANO Linear Response. The NEW SUPER SLR design makes use of a friction-reducing mechanism in the caliper, lever and cable that improve response and modulation.
Also for the lever, the SERVOWAVE mechanism is adopted to provide quick contact of shoe to rim and longer stroke from the initial braking effect for better control.
SLR is the essential factor in the SHIMANO dual-pivot caliper that deliver more braking power than conventional side-pulls.