Cracking your joints is called boiling or cavitation. When you crack your neck, the capsules around the joint will stretch.
These capsules have fluid, so stretching them will allow more fluid to provide less pressure on the joint.
When the pressure decreases the fluids on these joints will turn into gas. When this happens it will make a popping noise. This process is normally not harmful, especially when done by a Chiropractor.
The neck has facet joints. These joints can be located on the sides of your neck.
When you crack your neck, the facet joint stretches and lets the fluid spread in the joint capsules. The fluid becomes gas, and your neck joints will pop.
This is why a neck crack will feel like the pressure is releasing from the neck area.