mech. You don't stop the air delivery until it's completely removed from the tire or until you stop it. The automatic mode sounded like a big improvement, but it takes a little time and a few pre-sessions of tweaking it to be accurate. Once calibrated, you can get them to stop within a tenth of a psi of your target air pressure. I set them almost perfectly at 11psi on my 4900lb Jeep with 37" tires. Air pressure drops from 25 psi to 11 psi in about a minute to 1.25 minutes. The processing quality is very high. The feather is fleshy inside. The included pressure gauge is cheap but works. The packaging and green plastic box are unnecessary and bulky. They come in a pouch that fits in a glove box or pocket.