導火線 means “fuse,” the cord used to ignite an explosive. In the song, 導火線に火をつける means “to light the fuse,” vividly describing how anger and reckless energy build toward an explosion. It is a memorable word with both a literal explosive image and a metaphorical sense of triggering trouble.
Janne Da Arc's "Explosion" starts out as pure rage. The singer has been lied to and dumped, "欺かれて '見捨てられて'" [deceived, "abandoned"], until his heart and even his face are twisted out of shape. The anger rises "紅蓮の炎 湧き上がる様に" [like crimson flames welling up], and the first verse goes dark fast: "同じ苦しみを味あわせてやる" [I'll make you taste the same pain]. Then comes the bully's logic, "目障りなら '壊せばいい'" [if it's in your way, "just smash it"], and a flat refusal to talk anything through. The English shouts in the chorus ("Don't stop fury", "P.A.I.N?") make it sound like a fight breaking out mid-song.
Then the song turns on that anger, and this is the fun part. "弱い犬ほどよく吠えるから" [the weaker the dog, the louder it barks] is a real Japanese proverb, close to "all bark and no bite". The chorus mocks someone who confuses "'反抗'と'主張'" ["rebellion" and "making a point"] and asks "いつまで ガキのままでいるの?" [how long are you going to stay a brat?]. ガキ is rough slang for a kid, and キレる / ブチギレ mean snapping or totally losing it, so "闇雲にブチギレパフォーマンス" [a blind, blow-your-top performance] is a pretty savage jab at tantrums staged for attention. The wordplay "ハッタリキメmotion" [a bluffing, posing motion] squeezes "emotion" into "motion" for show. It ends on a surprisingly kind note: stop dragging old wounds around and use that same energy to "消せない 炎を飛び越えろ!" [leap over the flames you can't put out!].