I have never used this method, but I don't believe that water really flows out through the tubing because it is on top of the J tube (or U tube). A valve to shut off flow might be a good saftey measure though.
I always just shoved the airline tubing up one side of the tube and sucked the air out with my mouth, and sometimes some water. It is important to have enough water flow through the tube to prevent bubbles from collecting in the pipe though. The flow should be rapid enough to push air right through the tube.