Siphon is a daemon which, when triggered, reduces the agency's PWR by 2–5. The amount of PWR drained is random. If Siphon would drain more PWR than the agency currently has, then it is drained to 0.
Siphon is usually a relatively weak daemon; the cost of hacking a device with Siphon installed is usually equivalent to using your breaker program one or two extra times.
Siphon is most harmful when you are low on PWR but not at 0. To mitigate its effect, you can use up the PWR you have before hacking a device with Siphon installed, so that there is no PWR left for Siphon to drain. You will still be at 0 PWR afterwards, but at least you can get some use out of the PWR instead of just losing it.
When playing with Central, Siphon is strictly beneficial to the player, because it causes her Antiviral Proxy augment to generate at least as much PWR as Siphon drains.