[Veloist]

If you recall anything from a physics class, try to remember the topic of inertia.

When you're pressing the gas, the weight of the car & your passengers' bodies are all being moved forward constantly as you accelerate.

When you let off the throttle, the engine braking slows the car down and causes the weight & passenger bodies to continue to want to keep moving forward. This is what causes passengers to jolt forward.

The smoothest way to go about this is when you let off the throttle, don't just take your foot off. You want to ease it off so that the force of all the weight moving forward has time to slow down with the loss of speed.

With that said, it's all about legwork, and it's actually a very fine motor skill rather than a quick leg in, leg out motion--especially since you're not racing or power shifting. You're not going to get it right away, but each time you shift with a passenger, you'll be able to see in your peripheral if they jolted a bit forward. With them in sight, experiment your foot/legwork on the throttle release and you'll find the right way to take off the throttle.