The pinch test is basically just pinching the fuel line briefly to restrict fuel flow.

Get the engine running at a little above idle...maybe a click or two above idle on your throttle stick.

Generally speaking, if the carb is adjusted lean, then pinching the fuel line will cause the engine to die abruptly. (not good...too lean )

If the mixture is adjusted too rich, then pinching the line will cause the r.p.m. to rise quite a lot, then slowly die. (not good...too rich )

When the carb adj. is properly set...pinching the fuel line for several seconds, will cause the r.p.m. to rise a few hundred r.p.m. before it sounds like it wants to quit running.