When you set the index_offset property of an object in squirrel, the frontend goes off and loads the new image/video at that image offset right away so it is immediately available.
rawset_index_offset() is an optimization/hack that allows scripts to do essentially the same thing (set the index_offset), but without triggering any of this reloading by the frontend that normally happens when the property is set. This can be useful if the script knows that image loading is about to be triggered anyways (such as during a start navigation transition callback). I'm pretty sure the only thing using this currently is the conveyor module.
cheers
