An apparatus comprising: a shaft which defines and is rotatable about a first rotational axis; an axle defining a second rotational axis, the shaft extending through the axle; a first piston member provided on the shaft, the first piston member extending from the axle towards a distal end of the shaft; a rotor carried on the axle; the rotor comprising a first chamber, the first piston member extending across the first chamber; a housing having a wall which defines a cavity, the rotor being rotatable and pivotable within the cavity; whereby: the rotor and axle are rotatable with the shaft around the first rotational axis; the rotor is pivotable about the axle about the second rotational axis to permit relative pivoting motion between the rotor and the first piston member as the rotor rotates about the first rotational axis; and characterised in that the apparatus further comprises: a first magnetic guide feature coupled to the rotor; a second magnetic guide feature coupled to the housing; and at least one of the first magnetic guide feature and second magnetic guide feature comprises a magnet operable to magnetically couple to the other of the first magnetic guide feature and second magnetic guide feature to pivot the rotor thereby inducing the rotor to pivot about the axle relative to the first piston member. a shaft which defines and is rotatable about a first rotational axis; an axle defining a second rotational axis, the shaft extending through the axle; a first piston member provided on the shaft, the first piston member extending from the axle towards a distal end of the shaft; a rotor carried on the axle; the rotor comprising a first chamber, the first piston member extending across the first chamber; the rotor comprising a first chamber, the first piston member extending across the first chamber; a housing having a wall which defines a cavity, the rotor being rotatable and pivotable within the cavity; whereby: the rotor and axle are rotatable with the shaft around the first rotational axis; the rotor is pivotable about the axle about the second rotational axis to permit relative pivoting motion between the rotor and the first piston member as the rotor rotates about the first rotational axis; and characterised in that the apparatus further comprises: a first magnetic guide feature coupled to the rotor; a second magnetic guide feature coupled to the housing; and at least one of the first magnetic guide feature and second magnetic guide feature comprises a magnet operable to magnetically couple to the other of the first magnetic guide feature and second magnetic guide feature to pivot the rotor thereby inducing the rotor to pivot about the axle relative to the first piston member. the rotor and axle are rotatable with the shaft around the first rotational axis; the rotor is pivotable about the axle about the second rotational axis to permit relative pivoting motion between the rotor and the first piston member as the rotor rotates about the first rotational axis; and characterised in that the apparatus further comprises: a first magnetic guide feature coupled to the rotor; a second magnetic guide feature coupled to the housing; and at least one of the first magnetic guide feature and second magnetic guide a first magnetic guide feature coupled to the rotor; a second magnetic guide feature coupled to the housing; and at least one of the first magnetic guide feature and second magnetic guide feature comprises a magnet operable to magnetically couple to the other of the first magnetic guide feature and second magnetic guide feature to pivot the rotor thereby inducing the rotor to pivot about the axle relative to the first piston member.