Simply put.... it gets them compliant with the cap while minimizing their losses on the active roster.
There seems to be a lot of confusion about this rumored deal, but it's quite simple. TBL wants to get the likes of Cernak and Cirelli signed while shedding enough cap hit to get under the cap ceiling. By sending them Zetterberg's contract, TBL can use the benefit of LTIR to exceed the cap. And while I haven't calculated every dollar of this proposed deal, and have no idea what Cernak and Cirelli will ultimately sign for, the idea is that in combination with sending us Johnson and his cap hit, TBL will then become cap compliant without losing any good, young players.
Now, in helping them out by sending them Hank's contract, and in helping them out by taking on Johnson's contract, Detroit would have to be getting something pretty dang good in return. This is an absolute ideal scenario for the Bolts, but it's going to cost them. What, I have no idea, but I'd suspect it would involve at least one of their 2021 1st and their goalie prospect, Alnefelt.
In the main board topic about this, I did some rough calcs.
81.5 cap limit + 8.15 (10% offseason buffer) = 89.65 offseason cap.
Tampa's current Cap hit: 83.404
Tampa does the following
Sign Cirelli and Cernak to matching 1 or 2x2.5 deals to bridge out of this. (+5.0 cap)
Trade Johnson + non-cap assets for Zetterberg's LTIR contract and non-cap assets (-5.0 cap, +6.1 cap/LTIR)
Tampa Current cap usage before any additional moves: 89.504m
Tampa uses Zetterberg's LTIR as SOIR (season opening IR) and so have a "new cap" of 81.5 + 6.1 = 87.6m. They're still over the cap by 1.904m. So, in a couple paper transactions before opening night, they send down two roster guys that let them get the 1.05m in cap relief each. On opening night, they can get under the cap. Then, I think the rigors of an NHL season might actually take care of any other cap machinations. Like you'll get guys hurt or have the sniffles or what not. And at that point, they're not under the gun NEEDING to make a move. Making a move would let them breathe more easily, but they would not be under the sheer weight of it.
Also, maybe Cirelli and Cernak sign for less on a one year deal. Maybe you sell them on the fact that "we're nuts to butts against the cap this year, so we will talk long term for 21-22."
E: They'd send down guys to try to get the cap relief exactly as close to their new limit as possible.