Fob Detectors / Cellar Buoys
Fob detectors, also commonly known as cellar buoys (amongst others) are essential items to a draught beer dispense setup. 
 
Function
Fob detectors work by preventing air from filling the line once your beer keg has emptied. The float/buoy drops in its jar to prevent the line from filling with air or gas. This function dramatically decreased beer foaming, otherwise known as fobbing. 
 
Things to remember
When you change to a new keg, remember to pop the float back up using the button located at the bottom of the fob detector or your product will not dispense until you've done this. 
 
When you pop the float, also remember to bleed the top of the fob detector to draw the beer through the chamber and up the line. Just press the top of the chamber for a couple of seconds to fill.