You can push a string under the head of the spike to hold it down, just like a regular capo that only affects that string.
They're very popular for banjo. Earl Scruggs uses them. My "Gold Tone" banjo came with them installed under the fifth string at the 7th and 9th frets.
The reason they're so popular for banjo:
The 5th string on a banjo only goes from the bridge to the 5th fret. Let's say you want to capo up two frets. A regular capo works on the four low strings, but misses the fifth string. That 5th string is unaffected unless you've got a railroad spike to tuck it under at the 7th fret.
Step 1: The Spikes
The spikes are .033" square with little hatches to help them grip the wood better. The heads are all different. Pick through them for the spikes you want. You want spikes with thin heads.
That's a .032" circuit board drill bit I happened to have. "99" doesn't mean anything.
The guitar is a Martin Backpackpacker guitar I'm stringing like a banjo.
The two highest-pitched strings on a 5string banjo are both .009" thick, which is about as thin as a steel string gets. That's why the "thumb string" on a banjo starts out at the 5th fret, otherwise it would have to be either very thin or under tremendous tension.
Instead of adding a tuning knob halfway down the neck like on a 5 string banjo, I'm just going to add these railroad spike capos at the 5th, 7th, and 9th frets.