Not really sure. I don't believe I have ever heard of this concept. I know what LTB and NLTB incentives are, just never seen the concept of it being treated as a signing bonus over a five year period.
To my knowledge, void years are indeed the only way to do this, but I would think the cap hit for that would accelerate as soon as the void kicks in.
This guy's contract details have got to be the most elusive we have ever encountered. Everyday there is a new wrinkle we learn of. lol