It is said to have been used by the king when doing “perforcejagt“ (German = parforcejagd), where you rode the animal down until it collapsed, and then dismounted to give the killing stroke. [Perforcejagt was made illegal in Denmark in 1777].
I've seen a lot of later hirschfängers with a clam shaped nagel, though this example seems more disc-like.
Im wondering was the Hirschfänger early on basically a normal langes messer (crossguard and nagel), so the nagel was retained through tradition OR does it actually have a purpose when making the coup de grace on a stag?
I haven't been able to figure out when you have the first Hirschfängers, but they have been used basically to modern times.


Source: http://dkks.dk/hirschfaenger