Siren is an active generator program which, when activated, generates +3 PWR but raises the alarm tracker by one step. It has no cooldown, and costs no PWR to use, but cannot be used after alarm level 6. Siren can be chosen as a starting program, but cannot be acquired otherwise.
Siren is somewhat comparable to Burst or Wildfire — it generates PWR immediately, but its downside means it should normally be used sparingly. However, unlike those programs, Siren has no cooldown, which means in principle you can generate as much PWR as you need in any immediate situation, as long as you stay below alarm level 6. So it can be tempting to use Siren again and again every time you need more PWR, but this is a bad idea, because you will quickly get to high alarm levels when there is still too much left to do in the mission. Generating no PWR after alarm level 6 is also a significant downside; if Siren and Consoles are your only sources of PWR, then there is a finite limit on your total PWR use for the whole mission.
Since each step on the alarm tracker is equal to the cost of just waiting a turn, in some sense Siren is like having direct control over a passive generator, which allows you to generate PWR by waiting. If you use Siren every two turns, then the 3 PWR it generates matches the output of Power Drip for the same alarm cost over three turns. If you use Siren twice per turn after alarm level 2, then the 6 PWR it generates matches the output of Dynamo for the same alarm cost over three turns. But in these comparisons, using the passive generator would also give you more turns to move your agents and have items or other programs cool down. So if you use Siren regularly, it is much worse than a passive generator.
On the other hand, the benefit of Siren over other generators is that you have control over when you get PWR. Passive generators can cause situations where you spend turns doing nothing while waiting for enough PWR to hack something critical (or, for example, complete the Power Relay Station side-mission). In this case, Siren trades alarm steps for PWR more efficiently than passive generators. However, most loadouts will not face this problem often enough for Siren to be worthwhile.
In addition to having direct control over when you generate PWR, Siren also gives direct control over when the alarm tracker rises. This can be exploited to trigger the alarm level security measures at convenient times, such as spawning an additional guard when you are ready to sprint-toggle near the guard entrance, or having agents located on particular tiles without having to end your turn there. It can also reset the cost of using Mercenary, gain AP from Reactive Myomer in the middle of your turn, or generate more PWR with Archive Internationale's Network Siphon augment.