Исподтишка is a dramatic adverb that translates to 'stealthily', 'slyly', or 'in an underhanded way'. It describes an action done secretly, often with malicious intent, like an attack from behind.
In the song, this word creates a dark, cinematic image: "Исподтишка летят ножи" (Knives fly stealthily). It perfectly captures the theme of betrayal and paranoia, suggesting a danger you can't see coming from someone you might otherwise trust.
This song kicks off with some seriously dramatic scenery. The singer tells us, "Агент зимы поджёг листву / Весь мир горит, и я живу" (An agent of winter set fire to the leaves / The whole world is on fire, and I am alive). It's a powerful way to describe a feeling of total upheaval, where everything familiar is gone, and you're the only one left standing. He feels completely cut off, with "Без запасных аэродромов" (no backup airfields) and looking at city lights that are "больше не знакомы" (no longer familiar).
This destruction leads to a huge realization in the chorus: "Я никому не должен / И никому не верю" (I owe no one anything / And I believe no one). It's a blunt and total declaration of independence. He's decided that roots don't hold him down and he is "внутренне свободен" (internally free). This freedom comes from a dark place, though. He notes that "Мой близкий враг родней чужих" (My close enemy is dearer than strangers) and that it's easy to betray love, but impossible to betray emptiness. It's a tough, cynical worldview, but it's presented as the only path to true freedom.