When an unbirthed character is inside of the vagina during the act of vaginal penetration.

Note that this does not imply pinned by penis because the vagina does not count as a surface for the purposes of the pinned tag, otherwise all vore would be considered pinning.

If the unbirthed character also tries to penetrate the incoming penis in return, that would also be penetration interlock.

See also trapped in condom and in the middle.