According to Mark :
um, thats not how it is on my pump, its an inline backflow valve.

Then it's not an anti-siphon valve, it's a check valve.

I'm sure there is more than one type

The valve you describe will not prevent water from flowing back into
the pit but will prevent it from sitting in the pipe.

Sure it will prevent backflow (or at least MOST of the backflow).

If, as others surmise, the backflow into the pit is because the pit
is lower than the outflow, and it's siphoning back, a valve as I
describe (at the highest point in the line) will instantly kill
the siphoning action and let the water drain out in both directions.

If there's risk of freezing in the pit, the anti-siphon valve will
ensure that as little water as possible remains in the line.

If you use a checkvalve, then you guarantee that the line remains
full of water even if the pit is dry. Which is a recipe
for (eventual) disaster if the pit ever freezes.

In order to minimize the amount of backflow into the pit, and
at the same time make sure that the lines empty when the pump
isn't operating, you arrange things so that the high point of
the line (and thus the position of the anti-siphon valve) is
as close as possible to the pit.

In other words, run the line straight up from the pit to a height
higher than any other in the routing to the outfall, and install
the anti-siphon there. When the pump stops, then, you'll at
most get a couple feet worth of line draining back into the pit -
instead of siphoning back the whole line (or worse, that plus
the contents of the ditch) into the pit.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.