Phil
It is a counterbalance weight. Place it at 180 degrees to the heaviest part of the meat on the shaft, you can also adjust the length by sliding it up and down to have a lighter or heavier counterbalance weight.
The best method is to have it so it can rotate freely and then place the meat on the shaft and tighten up the forks. Put the rotiss shaft on the bbq and give it a turn, note the position of the meat, do this a couple more times - the heaviest part of the meat should always end up at the bottom. Place the counterweight in the vertical position with the weight uppermost and about 3/4 extended and tighten in position.
Check the balance again and adjust the extension or shortening of the counterweight distance to suit.
Does this make sense.
Cheers