I think it has the opposite effect … you have to work harder to breathe. The easier it is to breathe, and the more you are able to relax, the longer your air will last.
Umuntu is right, I just want to give a little more context....
Sometimes second stages have one or two kinds of adjustments:
One is Venturi control, which is meant to make the regulator a little less prone to a free-flow when out of your mouth on the surface (think: on a long swim-out). You move it into the "restricted" position to prevent the free-flow until you are ready to dive, then you move it to the unrestricted position so that the Venturi affect is such that the air is flowing straight into your mouth.
The second is a "work of breathing" adjustment. Turned in, it's harder to inhale. Turned out (or open), and it's easier to breathe.
As somebody said, sometimes these are "combined into one control.
To share my opinion....I've always preferred second stages without these features....although the Venturi control is minimally problematic. If you are on the surface a lot, you can be a little more careless with where the second is when it's out of your mouth if you are in the habit of flipping the Venturi "off." Over the years, some manufacturers have presented this feature as a "Pre-Dive/Dive" setting. (ScubaPro D-series regs for one). Very high performance regs can blast air like a banshee!
The Work of Breathing adjustment has always baffled me. I can think of no normal circumstance where I would want to make it more difficult to breathe. (to Umuntu's point) The operative word there is "Work" of Breathing. If you have to work harder, you'll use more energy and need more air. Turning UP the Work of Breathing to save air will likely cause you to use more air. The only credible scenario anyone has ever offered to me that makes any sense is that it's helpful to be able to detune the reg if the seal in the second stage starts leaking a bit during a dive. OK. Maybe.... But here's the thing. That adjustment knob works by tightening and loosening a seat in the second stage. That action of tightening and loosening causes wear, which contributes to leaks over time. SO the knob CAUSES the problem. Proper annual (or more) maintenance can prevent that, but leaving the think wide open is the best idea.
It used to be common for us to remove this adjustment from our ScubaPro G250s (effectively turning them into G250Bs) to reduce this failure point. A properly tuned reg without the adjustment breathes just as nicely as one with it.
I think the reason these features have endured is the same reason why you don't see a lot of stripped-down "work trucks" on the lot at your local Ford or Chevy dealer. People like options, and dealers and manufactures like to give people what they want.
Anyway, kudos for asking for information, and enjoy the diving on Guam. I was there for many years, and miss the easy and variety of diving at your doorstep. Also try to get up to Saipan and Rota if you have the chance. Of course, Palau and Chuuk are close too,,,,but get some more dives under your belt first!
Happy Bubbles,
Nathan Dent