From Sheldon Brown:
Shimano Linear Response, a series of friction-reducing modifications introduced in the late 1980's in the Shimano 105 group. The 105 SLR brakes (the best sidepull calipers ever made, in my opinion) incorporated:
* Ball bearing caliper pivots.
* Low-friction cables.
* Nylon spring bumpers.
* Reduced spring tension.
By adding a weak return spring to the brake lever, they were able to drastically reduce the tension of the caliper return spring. Since the cable was being pushed at one end and pulled at the other, a positive return function could be attained with a much lower overall spring tension. This greatly improved the "feel" and sensitivity of the brake. (This aspect of the SLR design was, I believe, copied from Dia Compe, which calls it "B.R.S.".)
This system was replaced by "Super SLR" which is Shimano's name for double-pivot brakes.
http://sheldonbrown.com/gloss_sa-o.html#slr