Who knows what is hanging on the endhose, and what is it good for?

Is it some sort of air shut off that pinches the hose to keep concrete from dribbling out?

Indeed an air shutoff clamb or what ever you call in in english  

Surprised you don't know these. It is almost standard equipment around here..

its an Air Cuff. Stops the flow of concrete so the boom does not drain out. Makes correlation tests within 1/2% or no change at all depending on how you do it. Can swing over top of things, and not make a mess. Nice to have when you want to circulate, and dont want to make a mess also. it works best on newer pumps that are already plumbed for the air cuff. You can add it to any year/model of a truck with a little wiring / air lines .. but you will not have the safety delays or anything like you do on the newer pumps. If you wire it on an older truck, you can have the air cuff activated (Inflated) and turn the pump "ON", so it can get dangerous.. the newer pumps when you hit the pump switch, they deflate and there is a timer that doesn't allow the stroke to turn on until the bag is deflated.. They are a great item to have though