It's the car shifting into a drive gear I'd assume. I imagine that unlike conventional torque converters where you can shift into N while driving without depressing the trigger on the shifter (or equivalent button) the reason you can't do that in our car is because the car uses some engine braking when slowing down and will shift into N at a stop itself. Then when your foot comes off the brake, the car shifts back into first and off you go. Hence the delay/shudder.I hate that shudder. Ir reminds me of starting a manual from a stop in third gear. Assuming you have an automatic, here my suggestion. Take your foot off the brake, wait for the car to get moving forward on it's own before you push the gas, when the car is moving then give it gas. It seems there is a pause between moving your foot off the brake and the car moving. Maybe it's that hill assist thinking about what to do? IDK.