The term tottenkopf means loosely " deaths head " which while having some unsavory connections to the German SS later was used to describe a familly of later 16th and 17th century cavalry helmets where the visor to a greater or lesser extent looked loosely like the face of a skull. There's a bit of an explaination in the arms race that goes along with this that needs to be layed out as to how you get here. The " prow form " visor of the earliest armets ( late 1420's early 1430's and continuing in used well into the 16th century ) and later close helmets, refering the the wedge shape of the visors,was purely a function the physics of the lance influecing armour design. Mounted men, made up of by and large in the 15th and a portion of the 16th century, meant the nobility and thier retainers facing thier noble opposities with the principle weapon of the day for cavalry charges, the lance. This pointed piercing weapon required a very steep angle to deflect due to its shape ( sharpely pointed specifically for impaling ). As guns became increasingly common especially as the 16th century progressed into the 17th both for foote soldiers and cavalry, lances were largely discarded except for a few " heavy lances " or " heavy currassiers" regiments. This changed the ballistic requirements or armour, as now after the discharge of firearms, swords and pikes ( pikes would be comming from below rather than at the the same height due to thier being an infantry weapon )would be the principle steel weapons which would be dealth with, so the visor no longer neede the very complex/expensive shaping of the " prow form " but could simply be a steel/iron hinged plate that covered the face entirely.The simplest form of " tottenkopf " is literally a pivot hinged steel/iron plate with a cut out for the eyes, and the mouth, often nothing more than two rectangles for vision and a small square, rectangle or diamond for the mouth ( there are numbers of these in Graz, i've owned two myself over the years. They were pretty crude ). Better examples tend to feature more stylized decoration including the visor, often as in the example above with a more anthropomorphic element, giving the visor more " facial features " ( I owned one at one point in which the visor had painstakingly been done to mimic a human skull, but due to the non functional nature of the gorget lames I suspect it was a 17th century funery achievement ).