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Overall view of the Skylab shoe-floor interlock. Basically, sections of Skylab flooring were composed triangular brackets and the shoes had a triangular insert which would protrude through the floor and twist to lock. Properly anchored, the astronaut could then exert force on objects without that pesky "equal but opposite reaction" from hindering him.

Here's a picture of the flooring in one part of Skylab (which, coincidentally, also shows the Lower Body Negative Pressure experiment.

The sign below the display reads

This is a piece of floor from Skylab with a pair of astronaut's work shoes. Since an astronaut floats around in the weightlessness of space, it is necessary for him to twist-lock his shoes to the floor before he can do any physical work.