wrapping is a problem because it causes pain and potential damage where it probably wasn't intended to. i think most of the strokes that have left unintentional bruises and/or broken the skin have been when implements wrapped.
and nic is right, that it's about aim, but it's also about positioning. w tends to stand too close to me, which means that a lot of strokes wrap if the implement is at all flexible. it helps her to aim better if she's about 18 inches to my left (she's right handed) when using an implement. i'm making these numbers up, since usually when i'm on the receiving end, i'm more concerned about the fact that i'm about to get a spanking than on where, exactly, w is standing. i just know based on an experiment she did last night (which i will write about elsewhere) that the extra distance greatly improves her aim with implements.
as for holding still... there are different ways to look at this. for me, if the pacing or something else about a spanking is too intense--not about how hard i'm being hit, but about where it's putting my mindset--i am really not able to hold still. for me and w, the solution has been to make sure strokes come at a pace i'm able to handle, because the fallout from a too-fast/harsh spanking isn't useful.
but there's also the fact that w has always been pretty firm about what kind of movements are or are not allowed. i am absolutely not allowed to try to cover up my butt with my hands during a spanking, and it's earned me several *hard* strokes with the cane when i have. so i grab onto a pillow and do my best.
positioning can make a big difference with how much i move around, too. if i'm laying on my stomach on the bed, especially if it's over w's lap, it's almost natural for me to roll out of the line of fire. but when i'm bent over a table or couch, feet on the floor, i pretty much don't move.
aka jigsaw analogy