What is siphon/antisiphon? do all vp shunts have these?

Siphon / Anti-siphon: No, all shunts do not have anti-siphon valves and there still remains some controversy as to whether they make a difference. In theory as you rise from recumbent or seated positions the shunt will "siphon" because of the sudden pressure change with elevation. These devices are supposed to prevent that and thus avoid over shunting, ie, draining too much fluid with resulting disturbances.