It's marketing garbage for the unsophisticated diver. It combines two separate functions: Venturi assist and cracking effort, into a single knob for divers that don't know the difference. It's now "Easy" and "Hard".
But those two adjustments are completely different. If I were doing surf entries, I might crank in the cracking effort AND the Venturi to prevent inadvertent freeflow until I get out of the waves. But my cracking effort stays as low as possible all the time during the dive (others have different opinions), and only gets adjusted in when I perhaps develop a tiny freeflow at the end of a service interval, due to seat impression. Increasing spring tension might save a dive for me there. Otherwise, I want my breathing as smooth as possible.
My Venturi is never off DIVE in single tank, tho' I sometimes use it in Pre-Dive in sidemount doubles, since I'm switching regs back and forth and have to worry less about mouthpiece position when I remove one and insert the other.
Otherwise, the only function for the Venturi adjustment is in deep diving when increased air density might make your lightly tuned reg too twitchy due to increased Venturi forces. Adjusting it slightly towards predive might smooth out air flow.
It used to be that Venturi was adjusted by the shop tech to give "just short of freeflow" with a brisk tap on the purge button. But now, computer aided design has made second stage performance more predictable, and the Venturi predive switch is just used to prevent freeflow in the boat or during a giant stride entry, after which it should be placed in the Dive position.
No, the MBS is IMO just stupid. But predictable.
For the diver who understands his equipment, the functions should be kept separate. Before, if you left your reg on Predive with only the Venturi adjusted, you'd avoid freeflow and could hardly feel any difference when you hit the water. Now, with the MBS cranked in to prevent freeflow, your reg breathes very stiffly until you unscrew it. Dumb.
But kudos to you for asking a good question!
