Generally when the traction is super high, people will use a harder insert to help support the tire better. It decrease forwards bite and side bite. Usually try to run as hard as insert you can and still have the overall bite you need.

Air gap is what controls the amount of forward bite you have. Bigger air gap means more forward bite, smaller air gap means less forward bite, but better support for the tire, and generally less tire wear. The air gap lets the tire squirm around more and conforms to the road much better. I like a large air gap on tracks with lots of slow corners and when the track is a touch bumpy, and I like less airgap on smoother, bigger, and faster tracks to decrease rolling resistance, and so the tire just doesnt fold over on it self in the high speed corners.

An air gap also lets tires heat up quicker, and they dont get to as high of a temp (because there is air fowing around inside the tire and stuff assuming you have big enough hols in your rims) Less air gap will generally take a bit longer to heat up, but they will get to a higher final temperature. I will usually think about it when I'm racing outdoors in really hot weather to keep the tires from overheating.

To change your airgap you get different inserts. Thinner inserts like the Yokomo 039 series and the Sorex type C inserts have a large airgap because they are thin inserts. The HPI, Sorex type A, and yokomo 037 inserts are thicker, and have thicker profiles, so they will fill the inside of the tire up more giving you less airgap. Sorex type B inserts fall right in the middle in my opinion.