This is a nice simple build for a quick release hitch for a bike trailer. Most commercial hitches will run you $20 and up. This is around $5 in parts if you go to harbor frieght for the air hose materials. $10 if Home Depot. What I can’t figure out is the hose female coupler, how he setup the bolt thru it.
http://www.instructables.com/id/Quick-Hitch-for-bike-trailer/