Ideally when you suck it up through a vacuum line, you want to add the 1/2 can as fast as possible w/out the car stalling. It will hesitate and drop rpm's as you're adding it, so you have to find the perfect mix. Then, when you're right about at the 1/2 can mark, you want to add enough to choke out the motor so that it does actually stall.

Now, I personally am too much of a p$$y to do this. I've actually done it once to my Jeep, but only b/c it's a 4.0 and those motors are bulletproof. I'm much to worried about the Audi, VW's or random other vehicles I've owned to pour enough in to stall it out. I usually just pour enough to keep it running and at 1/2 can I just shut the vehicle off. Your call