Red tabs on dual body regulator

[Lunkerking]

So I have a dual body regulator as pictured. What is the function of the red tabs above the regulator gauges? They seem to reset the pressure to zero.

When should these be used?

Thanks for any advice!

View attachment ImageUploadedByHome Brew1494714349.716396.jpg

 

[Stillraining]

Their only real beneficial use is to bleed pressure before resetting to a different lower pressure, say you want to go from a 30psi force carb to 12psi delivery pressure.
Another possible use is testing individual kegs for pressure..you can zero out your gauge and move on to the next keg.