The Drop-D capo listed here is designed to simulate drop-D tuning by covering the first 5 strings, leaving the 6th string open. If you put this capo on the second fret, you end up with a low E that's a full step lower than it would be with a regular capo, which gives you instant drop-D (or in this case, drop-E) tuning. You're certainly not bound to just dropping the 6th string a step; you could come up with an alternate tuning that involved moving the capo up several frets. Used in combination with a regular full-sized capo, you can get altered tuning anywhere on the fretboard. You could even, I suppose, flip the capo and try to drop the high E instead of the low E.