Why do I hear a "thud" when I put my foot into the boot?

The sound comes from the air escaping the bottom of the boot as your foot slides into the in-step. A good ‘thud’ means that you have a snug, proper fit and fill out the volume of the boot correctly.

However, not hearing the thud isn’t necessarily a dealbreaker! Depending on the foot and the boot’s last, you may not hear a thud even in a perfectly fitting boot. Ideally the boot should take some resistance to put on, after which your foot settles into place nicely.