What the guys are talking about is an adjustable old work box support or brace, often used to support a fan box and fan but also very nice for any fixture under 150 pounds.
The ones I like to use work like one of those basement jack posts with the jack screw on top. You cut your 4" hole in the bay between the joists then put the support bar up through the hole and lay it on top of the sheetrock or plaster centered on your hole. You then unscrew the bar with with your hand which expands the bar against the joists on either side. When you can't turn it any more with your hand finish up with your channel locks. Now you can do chin ups on it if you wanted!
The kit gives you a bracket, nuts and a U bolt for attaching a 4" round regular or fan box to the bar.
-Hal
