What does a phosphate remover actually do?

I have a 20 gallon tank with 55w of CF light that's been running for at
least two years. No matter what I did, I couldn't get the phosphates in the
water down. It was coming from my tap water, which was two high to read
with the test kit that I had at the time (it was at least twice as blue as
the highest amount on the test kit, 3 or 5 ppm.IIRC.)

Anyway, I finally got tired of all the algae, and put in some Hagen Phos-X.
The algae cleared up almost immediately, and has stayed away for two months.
I took the Phos-X out, and am going to wait and see what happens.

Now my question: Does the Phos-X remove the phosphates by absorbing it or
by combining it with something else? I guess what I also really want to
know is that if it is combining it with something else, do I have to worry
about that compound breaking down and releasing the phosphates again?

Thanks,
Bruce.