In a car with a manual transmission, the clutch pedal usually has a lot of travel. One reason for the large pedal travel range is to allow novice manual transmission drivers learn the foot-foot coordination between the clutch pedal and gas pedal. Learning the engagement point takes time, experience, and a little skill.
When a clutch pedal is depressed, it releases (or disengages) the pressure plate from the clutch assembly, allowing for gear changes. Timing is critical since not enough throttle when the clutch engages can cause the car to "bog" (or stall) and too much throttle can actually start to damage the clutch if not fully engaged. The illustration shows the start and ending point of a clutch pedal as it is being depressed.
There is a point when the clutch pedal is being released where the clutch will engage. This is the point where applying the throttle is ideal. After many, many gear shifts, it becomes easier and easier with experience to "catch" this engagement point. However, when people drive cars with different clutch engagement points, it's very easy to stall the car initially. Keen drivers will quickly adapt to the new engagement point and remember it.
As a driving style becomes more "spirited", the hand-foot-foot coordination must become more intense as gear lever change frequency increases as does the rate at which the clutch pedal and gas pedal are depressed. If one were to get overly excited, a gear could be missed because of a miss-timed clutch engagement (also known as "blowing a shift").
The advent of an adjustable clutch stop lets the driver specifically set the ideal point of reference where the clutch pedal stops just after the clutch mechanism disengages. Therefore, once set properly, the gas pedal may be applied immediately after the clutch pedal is released.
This lets the driver shift with one less item to worry about, actually forcing them to become smoother. The clutch stop works on all BMWs (except the new E46 models). Additional benefit is gained when pairing the clutch stop with a UUC Motorwerks Ultimate Short Shifter!