Apparatus is provided, including a sensor for sensing a phase of cyclic activity of a subject's body system. A tool modulator includes a gate configured: in a first cycle of the cyclic activity, to allow movement of a tool, in response to the cyclic activity being at a first given phase thereof, following the given phase in the first cycle and prior to an occurrence of the given phase in a subsequent cycle of the cyclic activity, to inhibit the movement of the tool, and in a second cycle of the cyclic activity, subsequent to the inhibiting of the movement, to allow movement of the tool, in response to the second cycle of the cyclic activity being at the given phase thereof. An accumulation facilitator facilitates an accumulation of the tool in the tool modulator, and/or an accumulation of energy in the tool.