I had to do a google search on that, had no idea what you were talking about. Those are for taking cuttings of a plant. Put that around a leaf node, fill it with some growing medium, and hope they root. For a while I was wondering just how strong your painkillers were.
Nope,, never used those. The closest I've come is to purposely bury a section of a limb and put a paver over it. In dry weather toss some water on there occasionally. Worked well on a forsythia.
I'd think your challenge is keeping the moisture level right. Not sure how easy crepe myrtles are to root.
I've seen the technique used, but not with a special device. What I saw just looked like an opaque plastic bag wrapped around the twigs & taped in place. That would undoubtedly be cheaper, there were about 10 of those on one shrub. I'll probably be trying something similar next year, in an effort to produce rooted hawthorne cuttings from a very vigorous bush nearby.