What you feel
Shin bang is a bruised, tender or sore patch on the front of your shin, usually on the upper half. You might feel it while you ski, on the walk to the lift, or the next morning. Some people see bruising.
This page describes what skiers report and what the model can show. It does not diagnose anything. If it hurts a lot, or the pain stays after you stop skiing, see a physio.
Why the tongue gets pressed
Your shin and the boot tongue meet every time you flex forward. In a good fit the contact is broad and gentle. In a bad fit it is a small hard patch.
Three things are most often involved. - Your foot is not held still. If your heel or ankle moves inside the shell, your shin travels further before it meets the tongue, and it arrives with more force. - A pressure point from the liner or tongue. A hard edge or a gap under the tongue concentrates the load. - Your stance. In the back seat, you push your shin into the tongue to stay up, harder and more often.
Equipment first
Skiers on forums nearly always get the same first answer: a posted footbed. A footbed that holds your foot neutral and stable moves some of the work away from your shin. After that come a softer or better-shaped tongue, a liner swap, and a check that the shell is not squeezing your lower leg.
What the model shows
The figure above puts the same skier in a boot with 8 mm of play and in a snug boot. The difference is in how late and how hard the movement arrives. The model does not measure bruising. It shows the timing, which is what a fitter wants to know.
What to try
Work from the boot up. Ask your fitter to look at the footbed and the tongue first. Then check that the buckles are not pulling the cuff hard onto your shin. Wear thin, smooth socks, and do not tuck the sock into a ridge.
Timing
Many skiers get this early in a season or in the first week in new boots. The liner is still firm and your shin is not used to it. Many skiers find new boots settle in a few days. If they don't, see a fitter.