Fooling a pipe into "thinking" it is shorter (meaning it behaves acoustically as a shorter pipe) involves changing its resonance . this can be achieved by creating new pressure nodes ie. a barrel band . the point at which it touches the pipe/barrel changes the resonance. ( same as a finger on a guitar string )
mechanically. if you support a pipe at one end you have a load at the point where its attached. long lever .
so if you put a support the barrel at 2/3rds or 3/4's that load point moves . so basically the load point has moved .the barrel thinks its shorter . and so has the finger on the guitar string .(the barrel band) which as then changed the harmonic note /wave .
try lifting a yard brush with one hand holding the end of the handle . then try lifting it again holding it at its balancing point .