What a brilliant invention. Every tool kit needs one.
Insert lethargic clutch / throttle / wire-pull brake cable into the rubber section of the clamp-block. One side of the rubber has a wide opening to accept the thickness of the cable outer. push it all the way in - usually to the depth of the chrome end section.
the other side of the rubber clamps tightly down onto the wire once you tighten down the knurled locking screw.
insert the thin pipe attachment of preferred can of WD40 / PTFE / silicon spray into small hole in rubber. Fill that parched cable with a few good bursts.
The oldfashioned way is to hang your cables up by the nipple and then drizzle 3-in-1 oil down the wire, but it's snails pace laborious. This little gizmo gets much better penetration and instantly. BP