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.
Step 2: Mark the Spot
That's on a right-handed instrument facing you, head up.
Here I am marking the spot with an awl.
Step 3: Drill the Hole
If it's too small the spike might split the wood. If the bit is too big the spike will be loose.
I like a dremel tool for these small sized bits. With a larger drill it's easier to break the bit.
Step 4: Pound in the Spike
If it turns, use pliers to rotate it before pounding it more.
It should be just high enough that the string can tuck under it.
If you pound it too far you can pry it up with a knife.
Step 5: And there they are.
They could probably be a little closer to the string (laterally).
Step 6: Pencil-Rubberband Capo
Add more rubberbands or get thicker ones if it doesn't press hard enough.
If your neck has no crown (totally flat) and the thick strings keep it off the thin ones, rub it lengthwise on the strings. The wound thick strings will wear into it and it'll hold all the strings down.
If your neck has some crown (transverse curve) just whittle on the pencil til it has the shape you need.
Earl Scruggs originally used this type of capo. source: "Earl Scruggs and the 5-String Banjo"
Step 7: Endless Possibilities
for very elaborate capo arrangements.
For instance this book on the subject.
It seems to me like the railroad spikes are a great way of doing this - the ultimate capo.
If you do a lot of "partial capo" stuff, let me know how you like the railroad spikes for that.