Downhole tool control

Downhole tool and method comprising an axially movable tubular piston 14 and a flow restriction 20 which cooperate to allow the tool to take a no flow configuration (FIG 1C). An intermediate configuration can also be achieved where an intermediate flow area is defined (FIG 1B). The tool is reconfigured to an intermediate configuration by flowing fluid through the tool at an intermediate flow rate. The tool is reconfigured to an operating configuration (FIG 1A) by flowing fluid through the tool at an operating flow rate which is higher than the intermediate flow rate. Moving the piston between a no-flow position and the intermediate position comprises a transitional stage where the piston and the flow restriction cooperate to provide a step change in flow area.