For mandolin players, one of them is pick direction. If I see a mandolin player's right hand going every which direction, I can move his playing forward a long way by just suggesting that he turn his right hand into a metronome. Go down on the downbeats, up on the off-beats.
When you're strumming, this is natural. When you're playing a melody, lots of folks start out alternating pick directions each note, instead of each beat, and then have to break a habit. To make this clearer, at the start of Happy Birthday, the right way is this
Hap py birth day to youbut they're doing this
Down up down down down down
Hap py birth day to youor something even more random.
Down up down up down up
If it's still not obvious what I mean, start nodding your head up and down to a regular beat, then sing the song and see which syllables come out when your head's going down, and which ones when it's going up.
If, however, it is obvious and you're doing it because it comes naturally, and you can't imagine why anyone wouldn't, watch fiddlers bow. They change bow directions for completely different reasons, on purpose, and no two fiddlers bow the same tune the same way. It's perfectly normal for a fiddler to play several notes on a single bow-stroke.
As a rule, American fiddlers are down-bowers, who'll kick off a part by pulling their bows, and use a bow pull to emphasize a strong beat, but I've seen lots of Cajun fiddlers up-bow, pushing their bows for their strong beats. I've seen left-handed guitars, but never up-pickers -- never anyone choosing to use their up-pick-strokes for their down beat. Wonder what it would sound like.
Mandolins are tuned like fiddles in standard tuning, so many fiddlers cross-dress as mandolin players. They already know what to do with their left hands; it's their right hands that come them a cropper until they learn the trick of letting their right hands just mark the beat.
As Pete Sutherland says to classical-violin players who're trying to learn to fiddle, "On the one hand -- that would be your left hand -- you have a great advantage. On the other hand -- that would be your right hand -- you have a lot of bad habits to un-learn."
