I think some of you might be mixing up the venturi and the cracking pressure adjustment.
The venturi adds more gas as you inhale by an injector principle. Having this open means that the reg will continue to freeflow when it starts as the gas flowing out will suck the diaphraghm in and keep the valve open. Most medium to high end second stages have this.
The cracking pressure adjustment puts more spring tension on the diaphraghm so you need more negative pressure to open the valve - breath harder. This is only found on "some" high end regs.
Basically you have three options in selecting a suitable backup reg. I've written some more about it here: http://dir-diver.com/en/equipment/backup_reg.html
Peter Steinhoff
The venturi adds more gas as you inhale by an injector principle. Having this open means that the reg will continue to freeflow when it starts as the gas flowing out will suck the diaphraghm in and keep the valve open. Most medium to high end second stages have this.
The cracking pressure adjustment puts more spring tension on the diaphraghm so you need more negative pressure to open the valve - breath harder. This is only found on "some" high end regs.
Basically you have three options in selecting a suitable backup reg. I've written some more about it here: http://dir-diver.com/en/equipment/backup_reg.html
Peter Steinhoff
