Feinberg, in No One Like Him, offers the argument along the following lines.
An illustration might help to explain the compatibilist view. Suppose I decide I want a given student in my class to leave the room. There are three ways I could accomplish this. I could literally grab the student and carry him out of the room. In this scenario, the constraint in operation on the student involves a force (me) exerted on the student that involves bodily movement, but not bodily movement that the student’s will in any sense made happen. Clearly he did not leave the room freely.
Or, I could threaten the student with a failing grade unless he left the room immediately. In this case, while the student does not really want to leave, yet on the other hand the student does not want a failing grade, so he decides to reluctantly leave the classroom. The constraint operating here is a force (me) that does not entirely remove willing by the student so constrained, although what is “chosen” by the student is contrary to what the student wished to choose. Again, the student did not leave the room freely.
Finally, I could perhaps point out the various factors that make it advantageous for the student to leave the room, though nothing I say threatens the student in any way. The student may not initially want to leave, for after all, I am a fantastic lecturer. Eventually though, I convince the student by reason and argument (without threats or warnings of danger if he refuses to go) to leave the classroom. In this situation, while the student did not initially want to leave, after considering all the pros and cons of staying or leaving, his desire to stay changed to a desire to leave, and the student acted on that new desire. In this scenario, a compatibilist will say that while the act (leaving the classroom) was causally determined, the student was not constrained to leave but did so voluntarily, in accord with his own nature, according to his own wishes.
To summarize, compatibilists hold that for every decision a person makes there are causal conditions playing upon that individual’s will so as to incline it decisively and sufficiently in one direction rather than another such that the agent could not have done otherwise, given the prevailing causal influences.