No, it opens the exhaust valve killing any engine braking you have.

With a closed throttle my bike slows down just fine all by itself. You shouldn't need anything more until you have to hit the brakes.

The decompression lever moves a cam - which then prevents the rocker arm from following the cam profile all the way down, and as a result the valves are held off the seats slightly....which allows the compression stroke air to just go out the exhaust.

This system is designed for start up, and as long as the valve is help open the engine speed will not be any faster than the electric starter motor can spin the engine.  The stock system only holds the decompression solenoid open for a very short period of time....maybe one or two engine revolutions.  Using the manual decompression lever at engine operating speeds most likely would not be good for the cams and rockers......instead of the rocker following the cam profile as it normally does - the rocker would lose contact with the cam....then as the cam came back around it would "hit" or "slap" the rocker pretty hard.

And as others have stated - you would actually lose the compression braking, as the exhaust valves would be open slightly and air could move in/out of the exhaust pretty easily.  Old hit/miss engines used this system to control engine speed - the exhaust valve was held open by a governor, and the engine could freewheel until the speed was slow enough that the governor released the exhaust valve.