... in this particular context, by deduction from the following reference (but you should examine the article yourself. There are two switches mounted on the circuit board and two corresponding pushbuttons fitted to the case):

Nachdem die Platine so weit fertig gestellt und nochmals geprüft ist, kann sie in das bereits vorgefertigte und bedruckte Gehäuse eingebaut werden. Zur Vorbereitung sind in die Gehäuse-Oberschale die beiden entsprechenden Schanierteile anzuschrauben und die Taststößel einzusetzen.
www.finger.de-web.cc/energie/elv.pdf

In general, a Stößel is a sort of plunger in a close-fitting housing (both usually cylindrical) that moves back and forth along its longitudinal axis (an engine tappet being a classic example).

In this case, I think it means a mechanical element that is fitted to the enclosure of an electronic device to allow a pushbutton switch fitted to a circuit board to be actuated using an external pushbutton.

Also see the following reference for a description of a different type of Taststößel (in this case part of a probe), including a drawing of the probe construction:

mic.uni-tuebingen.de/ftp/pdf/TAMIC.pdf