Chris,
The tube is fixed to the rear crossbar on the side facing backwards, directly underneath the piece of slippy stainless sheet. The idea being that it stops the sheet from creasing in a sharp line when the bow is pulled against it and that no matter what, there will always be a curve to the sheet at the point of maximum pressure, helping the bow up and over. A photo looking across and underneath the back of the trailer is attached.
I think that the "stop" brackets that prevent the swinging cradle from swinging fully horizontal are even more effective and for the best of all possible worlds (if that's possible with a trailer), I recommend combining both modifications.
