Actually, it can be done both ways.

Brief pinch.
With the throttle wide open lean for max rpm then back off 2-3 clicks and then briefly pinch the fuel line. The engine should speed up and then back down if it's set properly. If the engine doesn't speed up or dies then yoru still too lean.

Pinch close
WIth the engine at a fast idle pinch the fuel line closed and hold it. The engine should speed up and die in about 3-4 seconds. If it takes longer then the low end is too rich, if it takes less time or if the engine dies instantly the engine is too lean on the low end

Ken