The Sand bags

Tied to their ankles and adds force to cleanly break the neck. The hangman varies the rope length and weight in the sand bags based on the condemns weight.

The sandbags are used provide the effort needed to unlatch the trap doors beneath the victim's feet. Remember that the full weight of the convict will be on the door and the effort to unlatch it with a heavy load on it is high.

When the executioner pulls on the rings, it releases the sandbags which fall far enough to build up enough inertia to snap the latches on the trap doors open.

They are not used to augment the prisoner's weight. There is sufficient slack in the rope to allow the prisoner to fall far enough through the gallow floor to snap his neck cleanly causing nearly instantaneous death, but not so much slack that will cause decapitation (supposedly happened to Saddam Hussein). The executioner makes calculations based on the prisoner's weight to preset the slack in the rope to allow enough drop speed to break the neck cleanly. Too little slack and the prisoner suffers a strangulation death (which nearly happened to Marshall Cooper), too much slack and the risk of ripping the head off is high.

Edit: here's the complete scene on youtube. Go to 1:15 and watch what happens. We see the hangman pull the rings, then we see the sandbags fall, then a split second later the prisoner's drop through the trap doors. The last thing we see are their shackled feet with no sandbags attached to them. Also notice the sandbags are right next to each other, not spaced out like the prisoners are.

https://www.youtube.com/watch?v=mb6Zfh70liw

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