That two spring setup is what a mower wit a throttle control, the tab people are pointing to is bent down at the factory to give it a fixed rpm. Later engines do away with that assembly as well as the larger spring. Basically, if you bent that tab up that assembly would pivot back and for and you could adjust rpm like you would if it had a throttle cable.
The smaller spring is the governor that sets the maximum rpm of the engine. This is the one you want to adjust to slow the engine down or speed it up only if its running too slow or too fast. Its very important that the engine isn't allowed to run too fast for too long or damage could occur.
To adjust max rpm you do not and should not stretch the spring, you simply bend the little tab it attaches to either toward (to decrease rpm) or away (to increase rpm) from the carburetor. Stretching the spring will result in the spring wearing out prematurely.
Hopefully this clears things up.
Wrong. If the two springs are not adjusted properly the engine will surge. The smaller spring is only used to get the last 200 rpm for max engine speed. Also the carb doesn't have an idle circuit in the carb, therefore will either surge below a certain rpm, or the engine just dies. The engine is not engineered to be a variable speed engine. the carb is for fixed speed only.
To properly set the engine rpm, you remove the tension from the small spring and bend the tab on the large spring to 200 rpm under max engine speed, then increase the tension on the small spring to set the final rpm.