The anti-siphon valve is a very straight forward thing. It's a standard anti-siphon valve that all engines must have and they come in two variants. Most without the hose, and some (like ours) with a hose. The valve is there to prevent siphon effect in the exhaust, so it just sits at the top and has a check valve that will let air into the exhaust when the siphon effect otherwise could happen. The valves without a hose (just a hole) works fine 99% of the time. However, once in a while there are water or even steam coming out of the valve, and it is salt water! The location of the valve is right at the engine, so the salt steam/water will be sprayed right at the engine block, with nasty consequences.
The reason we asked Phil for this upgrade was that we met with one Outbound owner that had this happen and they upgraded the valve, so that spray goes into the bilge instead of onto the engine. Phil was happy to change it, and as I said, I think it's standard today.
Here is the link the the valve we've got: https://www.vetus.com/en/exhaust-systems/air-vents/antisifone-con-valvola-4839.html
The hose is around 45" long,. It ends up under the engine, close to the shaft.