It sounds like you have standard planetary tuners, but you may have Keith tuners. Take a look here, and scroll down to the picture: http://www.folkofthewood.com/page2161.htm No banjo makers install them unless requested by the player, as they are VERY expensive, and unless you play the few songs that require them, you may not ever need their special capability. They were developed for changing the banjo from G tuning to D tuning while playing. The right hand keeps playing, while the left changes the tuning. The tuner is set beforehand by the player so it will only go so far in either direction. You tune the G string to G, set the upper limit knob so it can't go any higher, then tune the same string to F#, and set the lower knob so the string can't go below F#. You do the same for the B string, but the limits are B and A. I don't know of a banjo player having more than two of them, but it's possible. Watch this: http://www.youtube.com/watch?v=0aYqshyDNrg Pay special attention to their left hands during the song, there's a good closeup at about 0:59. Earl Scruggs invented a low tech tuner in the 1950's to do this, and wrote a few songs that needed it. Bill Keith designed the higher tech model some years later, and still makes and sells them. I think they have a lifetime warranty for the original purchaser. If this is what you have, you need to find a banjo player who uses them to show you how to make them do what you need on a dulcimer. Since they are there, you may want to learn to use them. Once you know how to use them, you can use them like normal tuners, there's just some extra learning first.
If you have standard planetary tuners, set the screw in the knob for enough tension to hold the string in place, but not so much you have trouble tuning the string.
For those who may be wondering if Keith tuners are good to put on your dulcimer, they are reportedly very good tuners, but at the price of $220 per pair, I wouldn't recommend them on a dulcimer hoping to make retuning a little easier. They have to be set for specific tunings at the upper and lower limits, so you would have to loosen the set knobs for anything outside those limits. This would make retuning harder in some cases. If you set them to go from DAA to DAD and back, they would work well, IF they have enough range to go from A to D. I don't know if they have that much range, but if any one wants to try them, I would suggest contacting Bill Keith before buying them. $220 is a good start towards another dulcimer, which could be kept in another tuning. Re tuning ain't $220 worth of hard.
Paul