In Season 1 Episode 14 of Supergirl, when the detective reveals himself as the Master Jailer, he shoots Alex and then he points some kind of light at Kara. Then we see her waking up in that jail.
How did he so easily knock her out?
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According to this wikia article:
Additionally, the Master Jailer demonstrated the ability to fly, shoot energy blasts from his forearms, and project super-strong chains. Its energy blasts and chains where notably used incapacitate Kryptonians, along with several aliens, with ease.