Pushing in the brake release knob applies air, especially on dual chamber cans, to the bladder area opposite of the spring brake. It applies enough air to over come the spring. This is why, if you start losing air pressure, at about 60-65 psi or so the spring brake will start to apply.
The way you described it working is correct. Push in the yellow knob to release the parking brake. If you have a trailer attached, you push in the red knob to charge the trailer brake system. Once the air has gone to where it needs to go, you can start rolling.
