I can only imagine what Cohen gave them, but I sense it was explosive and major. And I suspect he had documentation to back it up. And I have to believe that anything he has the Special Counsel either already had or will have if Cohen newly discovered it.
Related to this, Cohen himself looked totally different from how we’ve seen him for the past year, which was scowling, under siege and imploding on himself. But yesterday, after the session, talking with the press, he looked like a major weight had been removed from his shoulders and was awash with relief.
By the way, lost in all this is that not only is Trump's longtime personal attorney Michael Cohen testifying against his former boss -- however it's not just that he merely "flipped," but even more: keep in mind that almost up until the FBI raided his home and offices, Cohen was telling people that Trump was like family to him and he would take a bullet for Trump. That's the man who is now so openly testifying against Trump.