Overview on how to use special Technic parts?
In general, the best strategy would be to look up instructions for a set in which the piece is used. 
Also a good strategy: ask here ;)

For the specific parts you mentioned, this picture might be helpful:



The 6641 element switches a driving ring in three positions, either engaging the left clutch gear (18946), the right clutch gear or neutral position in which neither is engaged... 

My favorite set these elements are used in is is 8043:



This set has 4 motors, however one of the motors is used to turn switches consisting out of the discussed elements to ultimately provide 6 functions: left &amp; right driving and turning the upper part on one position of the switch, while operating the digger (bucket, middle arm, lower arm) on the other position.