Tailstock pressure pad.

[mklotz]

Modelmakers often use this technique as the only means of securing small delicate parts for turning. Typically, the part is pressed against a faceplate with a suitable friction-generating material between part and plate. It works especially well for very thin parts that can't be clamped in a chuck without distortion. If they need turning work done on the circumference, this technique, perhaps augmented by a sacrificial piece of wood between part and plate, does the job nicely.