Based on their data collection, they predict that the 2 million males age 12-17 who have received the mRNA vaccines should have 0-2 cases of myocarditis. Instead, the observed 19 cases.
The risk is primarily after the 2nd shot of an mRNA vaccine for young males. They speculate on the mechanism for this. One speculation is that for some people, their immune system reacts to mRNA as a pathogen and attacks it, causing complications.
Their data sources are US hospitals, Israeli health ministry, and US military. They point out various flaws with VAERS, the CDC's database for adverse vaccine reactions.
They conclude that despite the elevated risk of myocarditis, the vaccines have a net benefit due to reduced hospitalizations.