.The right angle

Allowing for minor variations within each position, there are precisely six different positions that a person who is about to spanked can assume, and the most significant variation is the angle the legs make with the torso,

The first pair of positions is when there is no angle at all between legs and upper body:  the victim lies flat on the floor, flat on a bench or even flat on a table.   One variation has the legs together, and in the other, the feet are shoulder width, or more, apart.    Video buffs will recognize this position as being favorite of the Russian video producers:  flat on a bench or upright at the whipping post.

Now, there are two main areas of the bottom which get spanked:  the "meat" that is the center of each buttock, and the "under-hang", that area that lies between the crease at the top of the thigh, and the meat above it.

In this first position, whether legs are apart or not, all the spanks will land squarely on the meat of the bottom.   To all intents, the under-hang is effectively taken out of play, except for random strokes from an extremely flexible instrument.   Which is why this position is possibly less popular in real life than it is in some movie studios.

The second position is when the thighs are at right angles, or thereabouts, to the upper body.  This is the automatic position assumed by someone across a knee, or standing to lie atop a desk (or even the back of a buggy cart).  And in this position, both meat and under-hang are both readily available to whatever implement of choice is being used.   The whole of the bottom, from cheek top to cheek base is open to chastisement - and the legs together or apart simply increases the ability to land slaps on the more sensitive area of skin that lies towards the very center of the bottom.   When the cheeks are splayed, the flesh on the inside of each cheek turns slightly outwards, increasing the chance of each yelp being substantially louder.

And finally, we have the position when the angle is substantially less than 90 degrees.   In position 2, the head was level with the backside, now it is considerable lower.    The easiest way to achieve this position is "touch your toes" or its American counter-part "Grab your ankles".    But there are variations, including the simple kneel on a chair and put your hands on the floor.   One film studio has a spanking bench with a hole in its center for the head to be poked through - an out of the ordinary but quite effective manner of achieving position number 3.   And the diaper position is a variant of this, save the person being spanked starts by lying on their back.

The act of assuming position three flattens the under-hang to such an extent that for all practical purposes, the meat becomes largely irrelevant.   The under-hang has become almost the entire target to be punished.   The aftermath of paddling videos, particularly those that replicate school paddlings, end with buttocks thoroughly bruised from top of thigh to just under the meaty part of the buttock.   And this is the whole point of a touch your toes caning as well - the welts will lie across the lower, more sensitive part of the bottom.   Not only does the position make the penitent take the most submissive posture readily available, the punishment is slightly more effective than that received in position 2.   Again, legs together or apart simply increase or decrease the amount of central area flattened to be part of the target area.

As a spanking author, you will be able to engineer your characters into which ever position your plot suggests, and your only issue is how much time you will devote in your narrative to describe what the assumed position has done to make your victim more accessible to the torments, or pleasures, to come.

If stuck for a new plot motif,  you could try a spot of reverse engineering.   By whatever method, start by choosing one of the six positions.   And furniture, implement, clothing.   Then try to work out how these two came to be in this exact tableau and work outwards to what started it and what happened afterwards.   That she is stood spreadeagled at a T frame to have a dressage whip applied to her rump suggests all sort of plot devices than those that result in her touching her toes awaiting for a rattan cane to reduce her to tears of pitiful remorse.

Not a cure-all, but one tool that you should have in your writer's bag, and understanding the anatomy of the six basic positions will avoid you describing an impact that simply could not have happened in real life.