The sausage's signature snap when you bite it is basically the point — 'knacken' is German-Alsatian for 'to crack.'
Knack is a thin, all-beef or pork sausage in a natural casing that stays taut and snaps audibly with the first bite, simmered hot and slid into a length of baguette with mustard. It's a fixture of Strasbourg's winter market stalls.