"Nahverteidigungswaffe" or "Nahverteidigungsgerät" was, at its simplest, a rotating firing port mounted on the commander's hatch, through which the crew could fire a frag grenade with a flare pistol. Unfortunately, holes in armour work both ways, so the port was often welded shut, and the effectiveness of firing a single very small grenade essentially blind was questionable anyway, especially if the enemy had 'zooks or PIATs. Equally random was the habit of mounting remotely detonated S-Mines on the outside of the tank - to know when to detonate them would have required the crew to see the attacking infantry, and if you can see them, you should either shoot at them or drive away...
tl;dr: practically useless IRL and not worth the effort to implement in-game.
