I've tied some damsel nymphs with extended body glass beaded bodies...experience was as Buddy suggested ...not much motion.

Perhaps the weight of the metal beads drops the fly in the water column and causes the undulation as the originator suggests .

I'm always looking for more motion. I checked my bead chains ...the plastic ones I have [on string] are stiff ...the metal ones do have quite a bit of flexibility...[except the ones I dipped in fingernail polish to color].