A drill string tool has a control mechanism to switch the tool between an inactive condition and an active condition in response to operator-performance of a predefined trigger sequence comprising variations in a drilling fluid pressure differential. The trigger sequence comprises multiple cycles of (a) raising the predefined trigger sequence into, but not above, a predefined pressure range, and (b) lowering the predefined trigger sequence below a lower threshold of the pressure range. Raising of the pressure difference above an upper pressure range threshold results in automatic interruption and resetting of the predefined trigger sequence.