WARNING: The following contains spoilers for Boruto: Naruto Next Generations Episode 194, "The Uzumaki Household," now streaming on Crunchyroll.
In the Boruto manga, the flashbacks on Kawaki's time as a test subject in Jigen's lair were a bit light. That's why fans are really appreciating the current filler in the anime as we're getting to see a lot more of his origin, in particular, how he was sold to Kara so Amado could experiment on and weaponize him.
Now, Episode 194 of the anime gives us even more insight into his tragic past; specifically, the horrific truth about how he got his Karma mark while he was kept as a guinea pig in Jigen's sadistic lab.
In the episode, and at Naruto's home, Kawaki details his ordeal in Jigen's liquid chamber, which involved having masks hooked up to him. He's doing so because he wants Boruto to join him in finding a way to remove their marks. Seeing as he thinks Boruto's a clown, Kawaki's as blunt as ever in the hopes of shocking Boruto into becoming serious.
In the flashbacks, Kawaki is in the stasis tanks (conscious) as Jigen talks to Amado about how they've lost over a dozen test subjects. Jigen is frustrated as there are only three left, but he proceeds with another test run rather than exercise caution and patience. He activates his abilities through the black Karma tattoos appearing all over his body, then connects to a tube and the black marks turn into goo, akin to Marvel's Venom symbiote, draining through the tube and into the tank. There, it engulfs a teenager, flowing up into the person's nose and filling the entire tank, but Jigen quickly breaks the connection as he senses the person has died.
He then smashes the transference hub, and the bunch of other cracked ones around the place highlights how many died so far. As he floats in his tank, a submerged Kawaki is clearly scared hearing Jigen and Amado talk about how they're doing another run. Time is running out, leaving Jigen desperate for a "vessel." The boss looks up to a wide-eyed Kawaki, intimidating him and making it clear he cannot fail the absorption process.
Jigen then does the same, before letting the goo flow into Kawaki's tank and his orifices. The entire tank turns black, leaving him suffocating and screaming. At this point, the episode cuts back to Kawaki telling Boruto how he barely survived; the ordeal leaving him scarred by the poison injected into his mind, body and soul.
Boruto's curious about the black bands because he didn't get his Karma through this process -- he got it from Momoshiki. Nonetheless, he now wants to get to the bottom of things and is ready to accept Kawaki as an ally, feeling real sympathy towards him now.