an apparatus and method for performing high speed interruption of the flow of a very fine , high pressure , high speed water jet 12 of the type used to cut foods , paper , and other goods . the water jet 12 is interrupted by a pivotal blocking bar 22 within a blocker housing . the blocking bar 22 is pivoted in a collar 24b to a first desired position out of the path of water jet 12 or to a second desired position for blocking the path of water jet 12 . a pivot arm 28 , controlled by an output shaft 30 of a rotary actuator 32 , controls the rotation of the blocking bar 22 . a high pressured airflow is introduced into the device for controlling the exhaustion of blocked water within the device and for cooling the rotary actuator 32 .