Sinn’s tegiment process is a form of case hardening using gas carburizing technology. (Refer to here and here for more details). It is well known in metallurgy that steel can be surface hardened by the addition of carbon to the steel surface, a process called carburization. However the traditional carburization process when executed on stainless steel, reduces the chromium content which compromises the corrosion resistance of stainless steel.
Sinn is believed to use a more modern process in which the metal surface is first activated and carburization is done at a low enough temperature so that chromium content of SS is not lost through chromium carbide formation. This process allows Sinn tegimented watches to achieve surface hardness up to 4 times that of the steel core (1200 Vickers) without compromising on the corrosion resistance of the underlying SS. However the hardened surface is extremely thin ranging from 10micron to 50 microns. That leaves the case core beneath the hardened surface still remaining at the relative softness of the SS316L.
While the tegiment process makes the surface more resistant to light scratches, the watch is still not imprevious to hard knocks or dents. When a surface force is applied which exceeds the hardness of SS316L, the steel core will give way and a deep scratch will still occur despite the hardened surface. This is also commonly called the egg shell effect, because egg shells while being resistant to light scratches will still crack under pressure.
On the other hand, Damasko uses a different process called ice hardening in which the whole watch case structure is hardened, instead of just the surface of the watch case. The ice hardening process normally involves heating the steel to its normalising temperature and quenching quickly to a sub zero temperature. Ice hardening changes the crystal structure of the steel itself, thereby hardening the whole watch case, albeit to a lower hardness of approx 800 Vickers. By hardening the whole case, the ice hardening process, in theory, eliminates the egg shell effect exhibited by tegimented watch cases. It is hard to say how ice hardened watch cases perform in the real world because Damasko watches are not the easiest to come by.