"Arm-turning pin" or "second form".
Nikyo builds on ikkyo (first form). It can be understood as a follow-up technique that is used if ikkyo does not work, e.g. when tori fails to stretch uke's arm.
The characteristic element of nikyo is to turn and fix uke's wrist by applying pressure on the base of the thumb.
The nikyo immobilisation pin is distinctly different from the ikkyo pin.
Classic form by Christian Tissier Shihan:
Alternative form by Endo Seishiro Shihan: