By going into the crawlspace they would probably be attempting to tighten the boards by way of using screws. It's a good idea. What occurs is someone finds the affected areas from above, stands on them allowing weight to snug the board up to the subfloor. Then the one from below drives in some screws so it's secured. Finding where to insert the screw from below is the hardest part.
Here's an older video (link below) I put together about the epoxy idea. It gets the point across but ain't the prettiest. My first ever video. We've discussed it before and some guys doubt the procedure including myself because of the epoxy properties. But it is performed by the "go to" guy at the National Wood Flooring Association and he has a lot of respect worldwide. I cornered him at one seminar and expressed my doubt about the epoxy creating an expansion problem. His reply was along the lines of ..."if the flooring doesn't remain in a safe humidity range, it's not my problem." In other words, it's in a perfect world. Most of his work is in the high end area where people use strict climate control. I agree with Stephen on the adhesive type.
http://www.youtube.com/watch?v=PjyjHuvdXDs