That sauce is generated by the brown sugar, honey, little apple juice sprayed, and the juices of the ribs. What I do is baste the ribs in those juices that are laying in the bottom of the pan that I use to steam the ribs in foil.
If you recall, I place the brown sugar, and honey in the pan, take the ribs from the grill and place them meat side down. Then I spray the bone side up with apple juice.
I think that this helps to keep the meat from burning as brown sugar, honey and the juices of the ribs liquify.
Ed