This is a very basic guide - A Vespa crankshaft differs from a Lambretta because part of the crank controls the induction of the gasoline/air mix into the crankcase. On the clutch side engine casing there is an intake hole directly below the carb. This passes through the casing into the crankcase and the interior face of the hole is perfectly machined to fit against the circular clutch side crank web. Part of the clutch side crank web is machined away from the circumference of the rest of the crank half. This means that as this area rotates under the induction hole, it becomes open and at the same time the piston is rising up, creating a vacuum in the crankcase. Gasoline is sucked into the crankcase because of this vacuum. To a point, the longer the crankcase valve is open, the more gasoline/air the engine can suck in.

Below is a model of a P-series crank showing the machined clutch side crank web nearest to you, and the point where it has been machined down to allow gasoline to be sucked through from the carb. You can see this web is perfectly circular except for the cut out just above the crank pin.

When people talk about having a "cut crank" they are referring to machining this cut out to have less of the circular crank edge and more of the opening which allows the carb intake to be open for a longer period of time in every rotation. This intake time is measured in degrees.

The red area above is the area which could be removed from an existing crank. This involves having your crank split by a motorcycle/scooter shop because there is no way to protect the main con rod bearing from all the waste metal that will be ground out.

Many manufacturers of aftermarket cranks sell cranks with an opening that is machined to allow the intake to be open for more degrees of rotation. A standard crank is open for about 155 degrees of the 360 degree revolution. A heavily modified crank can be open for as much as 200 degrees. If the crank is opened up too much you will allow either the piston to be coming down when the valve is open which will force all the gasoline/air back out the intake (your motor won't run) or in the other direction, the transfers may be open at the same time as the inlet which is also a bad thing.

Above are shots of a Mazzuchelli crank we bought for a Malossi P210 motor on the left and a standard 200cc (in sad shape) crank on the right. On both cranks the inlet opening is at the same point near the big end pin. The Mazzuchelli crank comes already cut to allow for a longer inlet period as noted with the small red arrow. Some other small differences are the shape of the intake lip is angled on the cut crank, the window in the clutch side is smaller, and the crank surfaces have been polished for gas flow. Click on images for a larger view. Below are images of a small frame crank with the same treatment.