The expanding stem caster expands inside the leg when you turn a nut on the bottom of the caster.
To start the installation, there is a round knurled nut on the top of the caster stem that you turn until it you have an initial snug fit when you insert it into the leg. Once you have a snug fit in the leg, you then turn another nut on the bottom of the caster (see photo), which makes the rubber inside the leg expand. It takes less than 5 minutes to install once you have cut the legs off. Make sure the measure the inside width of the leg so you order the right size. Mine was 1 inch inside diameter and I ordered the 5 in wheels so it would roll easy with one wheel a locking wheel and the other a regular wheel
View attachment 1487. They have all sizes and wheel composites to choose from. I got the non marking ones.