Plank

When a slut planks or is planked, all of her muscles below the chin become as stiff as solid steel—indefinitely. This also skyrockets a slut's defense, allowing her to withstand far more damage than usual, but doesn't necessarily rob her of sensation. One's face is still fully expressive. This position is particularly useful for wielding my body as a bitch-blade, riding me as a slutboard, or using me in cuntstructions.~ Any slut can choose to plank herself, or You can command a slut to plank—either directly or through the effect of a stunt.
  (As with most things, this position is only possible through sustained hyper consent. Even if we're pretending to be "forced" into this position, our superhuman ability to maintain it is borne entirely of insurmountable desire to sub unto You. :3)