Coaxial valves have a single moving part: the control tube (a).
When the control tube is actuated by an electromagnet (Figure A) or a pneumatic / hydraulic signal to the actuator (Figure B) the tube moves away from the valve seat (b).
This allows the media to flow straight through without deviation. Powering the tube back against the seat closes the valve. The tube is held against the seat independent of system pressure. This provides positive shut-off and compensates for seat wear. Dynamic seals (c) prevent shaft leakage.