Blue slider can be positioned at various positions during going up thanks to yellow lever of pink pin and the gravity.
Red spring forces the pin towards the teeth of white runway. Small slot on the yellow lever prevents the pin from contact with the runway teeth once the spring enters the slot.
Slanting portions at the ends of long slot of the runway control the meshing of the spring and the slot of yellow lever.
At highest position of the blue slider, the pin is separated from the runway teeth. The pin does not contact the runway teeth when going down.
At lowest position of the blue slider, the spring is pushed out of the lever slot and the pin now can contact the runway teeth.
So the engagement of the pin with the runway teeth is controlled automatically.
To get higher position of the blue slider: raise it until it locates into desired holding position.
To get lower position of the blue slider: move it as high as possible, then move it down as low as possible and raise it until it locates into desired holding position.
This mechanism can be used in
   • Cam-Stay - Multi-positional telescopi...  

STEP files of this video:
http://www.mediafire.com/file/oh4p9rx...
Inventor files:
http://www.mediafire.com/file/uhq88fs...