I know the solenoid is either on or off depending on whether voltage is applied or not.

So how exactly is the ECM commanding, say, 50% (just picking a number) duty cycle on the CCP at a given moment? Is it simply enabling the solenoid on 50% of a given length of time, then shutting it off, then enabling it again for a given length of time, then shutting it off, etc...

 

Pretty much. The duty cycle is defined as the percentage of time "on" as compared to a "full cycle" of a given time period. A 50% duty cycle would be "on" for a given period of time, then "off" for the same amount of time. A 25% duty cycle would be "on" for a period of time then be "off" for 3 times as long and so forth. The total time for a cycle is irrelevant to the duty cycle as the duty cycle is just a percentage of "on" time for the total time period, whatever the time period may be. HTH!