Most Sirens you encounter in coop are using Res, and the ones that aren’t are questionable, but sometimes it is better to Phaselock an enemy rather than Res an ally. For example, your teammate is surrounded by a trash mob, a second wind is almost inevitable and if you use Res it will be a wasted cooldown. There’s no need to panic and hit Res every time an ally goes down, look around and assess your surroundings before throwing out a Res! If you can, draw aggro and apply slag or weaken an enemy so your teammate can get themselves back up (though Cloud Kill does like to steal second winds), you’d typically do this with other characters so why not Maya? Res should be a last resort revive, not an unnecessary panic button that leaves you with no slag or CC on enemies.
And that brings us to slag and CC. Sometimes it’s better to Phaselock an enemy - either the closest enemy to your downed teammate or use a singularity grenade to pull one close - proccing Ruin and Converge, and let your teammate get up that way. Ruin and Converge are invaluable, wasting them on a revive when you could just as easily use them to your advantage to allow your teammate a second wind and set up the next chain of kills could prevent a chain of fight for your life, so think before you Res! I see far too many Sirens throwing out Res as soon as an ally goes down, even if they have virtually no chance of not getting a second wind the Siren will still use Res for no apparent reason. It’s not a panic button, fellow Sirens, it’s a last resort, use it as such, Phaselock itself is a utility that can be more useful on an enemy than a downed teammate if used right. Unless your teammate has had several consecutive downs and has a fast FFYL times, look around and assess your options before jumping to the obvious yet often unnecessary and detrimental one.
I’ve been practicing this technique a lot lately with forum members @Sun_Tsunami, @Codarik, and @Ginger_greninja, it’s very simple yet can be extremely effective and can make the difference between a successful clear and a FFYL chain leading to eventual death. I’ve been mostly a solo player for the last 4+ years and when I did coop I too was once a panic-Reser, but having focused more heavily on coop for the last few months after joining the forums I’ve learned this basic technique and it’s one of the most effective and efficient tactics I’ve ever used in Borderlands.