The Devilmite Beam transforms an opponent's own negative thoughts into destructive beams that can annihilate anything in their path. However, it only works if there are negative thoughts in the target's mind to begin with. Due to this, outside of the video games, it is an all-or-nothing attack.
That's from the wiki and yeah it just looks like it transforms bad brain signals?
https://dragonball.fandom.com/wiki/Devilmite_Beam
Also Roshi's explanation:
"It is a force of unimaginable evil, that has destroyed more lives than any war or pestilence combined. Long ago there was a mighty-but-compassionate warrior. His power was beyond measure, yet many foolish upstarts tried to destroy him, in hopes of making a name for themselves. They were all unsuccessful, until one day, the Devilman appeared. Knowing he could not compete against the warrior's strength, the Devilman attacked using his mind. He discovered he could exploit his opponent's negative thoughts, no matter how faint, and convert them into an energy beam of immense power, the Devilmite Beam. It amplified any dark thought, anger, malice, aggression and caused it to expand until his opponent's heart, literally exploded! It was the death of a great hero, and the birth of an even greater evil."
— Master Roshi's explanation of the Devilmite Beam
Seems to just use emotions as power.
Also Toriyama said that the attack wouldn't hit if aimed at future villains. Does that mean if it hit them it's gonna do a crap ton of damage especially if destroying their heart is fatal? Could Devilman oneshot Black?