Squirming, wiggling, wriggling, writhing means to move in a twisting or contorted motion, especially when struggling to escape a discomfortable situation. Squirming can involve every body part in any possible direction: the torso, legs, arms, and neck/head.
Squirming of a spankee
Squirming is common for a spankee, especially in the non-consensual punishment of children. Squirming can make the spanker's job difficult as he/she has to spend much energy on holding the spankee in position. Squirming can also make the spanking more unsafe if an implement other than the hand is used. If the spankee does not remain still, a blow that was directed at the buttocks may accidentally hit a wrong body part, such as the tailbone or the hand.
Some spanking positions make it easier/more tempting for the spankee to squirm, while other positions make it more difficult. Notably in positions where the spankee is standing, bent-over, or half-standing, the spanker will often explicitly instruct the spankee to hold still and not to get out of position while the punishment is administered.