Rolkur is hyperflexion of the neck. So instead of the poll being the highest point of horse, it is overbent with the 2rd or 4th vertebrae being the highest point. Often the horse will have it's head jammed into it's chest, thus the hind legs are unable to come up underneath him. Supposedly it is to soften the jaw, but essentially it is just pulling the horse down into a frame, also known as 'forced helplessness'.
I have no problem with putting a horse deep for a few strides if it is bracing the neck and jaw, however it is not a forced action as rolkur is.