A medical system for treating tissue, comprising: an elongated shaft assembly having a longitudinal axis and including a first member mounted to rotate, and optionally axially translate, relative to a second member, wherein the first member has at least one default position and other non-default positions relative to the second member; a magnet (840a, 840b) coupled to rotate with the first member; a sensor coupled to the second member and configured to sense rotation of the magnet (840a, 840b) as the first member rotates and configured produce rotational position information; a motor drive adapted to rotate the first member relative to the second member; an electrode mounted on or otherwise coupled to move with the first member; a negative pressure source in communication with a lumen in the elongated shaft assembly; and a processor configured: to receive rotational position information from the sensor; to control the motor to rotate and rotationally position the first member relative to the second member; to stop rotation of the first member relative to the second member in response to a predetermined event and to rotate the first member to the default position of the first member relative to the second member in the event that the first member is not initially in the default position, the predetermined event comprising activation of the electrode or actuation of a negative pressure source; to modulate or terminate activation of the electrode or to modulate or terminate actuation the negative pressure source, in response determining that the first member is inadvertently in a non-default position, and to re-activate the electrode or to re-actuate the negative pressure source after moving the first member to the default position. an elongated shaft assembly having a longitudinal axis and including a first member mounted to rotate, and optionally axially translate, relative to a second member, wherein the first member has at least one default position and other non-default positions relative to the second member; a magnet (840a, 840b) coupled to rotate with the first member; a sensor coupled to the second member and configured to sense rotation of the magnet (840a, 840b) as the first member rotates and configured produce rotational position information; a motor drive adapted to rotate the first member relative to the second member; an electrode mounted on or otherwise coupled to move with the first member; a negative pressure source in communication with a lumen in the elongated shaft assembly; and a processor configured: to receive rotational position information from the sensor; to control the motor to rotate and rotationally position the first member relative to the second member; to stop rotation of the first member relative to the second member in response to a predetermined event and to rotate the first member to the default position of the first member relative to the second member in the event that the first member is not initially in the default position, the predetermined event comprising activation of the electrode or actuation of a negative pressure source; to receive rotational position information from the sensor; to control the motor to rotate and rotationally position the first member relative to the second member; to stop rotation of the first member relative to the second member in response to a predetermined event and to rotate the first member to the default position of the first member relative to the second member in the event that the first member is not initially in the default position, the predetermined event comprising activation of the electrode or actuation of a negative pressure source;