Gravity and surface tension work together as blood flows from the wound.
View attachment 727149 Might the blood flow look something like this? (My artistry is awful!)
I held my arm up and imagined a nail thru my wrist. The blood (water) seeks the lowest level, so it flows to the underside of my arm following the contours of my arm. Then to armpit and down side of body. Seepage from the back of my wrist would follow the contours of the crossbeam (appears rounded n this image) and at some point, sooner or later, would drip to the ground.
Any competent executioner would not want the condemned to die too soon from blood loss. A nail would be pounded so that the flat head pressed firmly against the flesh of the wrist to seal the wound. (Hey, that's basic crux practices, right?
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If flat-headed nails were hard to make, I imagine the executioner might slip a thick piece of leather (like a washer) over the shaft of the nail so that the nail head would press it tightly against the wrist to minimize blood loss. The same idea applies to the feet.
The movements of the crucified person would also be a factor in blood loss/flow.