A control mechanism for a drill string tool is configured to activate the drill string tool by hydraulically actuated movement of the switching element to an activated position, with drilling mud serving as actuating medium. Movement of the switching element to the activated position is automatically regulated, so that tool activation is conditional upon application of above-threshold downhole drilling fluid conditions for at least a predetermined switching duration. A switch regulator that regulates movement of the switching element to the activated position can be configured to regulate a rate of movement of the switching element such that a substantially constant switching duration is maintained regardless of fluctuations in the magnitude of an actuating pressure differential during above-threshold downhole drilling fluid conditions.