60Dan, the protrusion is the nub at the end of the axle in the center (or the "center cap" if you will). With a hub centric, the rim fits tightly around that hub and is kept perfectly centered via that tight fit. A non-hub centric axle/rim does not care about that fit and instead centers the wheel via the lug nuts (the nuts usually have a taper to them that seat into the rim, thus centering it).
There will be no protrusions on the spacer. Some vehicles are hub centric from the factory, but then people throw on aftermarket rims with larger center bores (these rims are made to fit many hubs, thus they are made larger). In that case the axle/rim is now non-hub centric, which is fine, as long as the tapered lug nuts seat inside the rim's bolt hole taper.