Disc drag = drag of the prop (Think FF rubber, windmilling on glide, vs stationary prop, vs folded etc) at some point the prop will be causing drag as the airframe speed 'catches up' to the prop. The point where the prop shaft feels no 'pull' is where the prop disc thrust and drag are equal. As you said, per the single engine training on the 748, 'a bit of extra idle so there is no drag from the prop'. Just balancing the prop disc drag with a bit of thrust, so, being pedantic, there is still thrust being developed by the prop, it's just not being transmitted to the airframe.
Evan.