I believe that disables the 'Spring Opening' feature. The 'Wrist Breaker'.
But it's just a guess as I have never worked on one.

Self Opening/Spring Opening are Ok but you have to cock the spring(s) that is going to assist-open the bbls for you at some point. That is generally when you close the bbl's .
Some don't care for that heavy closing feature all the time, so the ability to disable it and have a peasent style SxS while no one looks on can be of use.