See https://www.youtube.com/watch?v=LbH1ZDImaI8 or https://www.youtube.com/watch?v=Owji-ukVt9M

But if I understand it right, the temperatures when you drive a propellant turbine can get so hot they melt or otherwise break the turbine, so to get around this they normally burn the propellant in the preburner in a less optimal way with either too much oxygen or too much fuel.

The full flow engine however does both at the same time using two preburners and two turbines and the combined fuel and oxygen mix that reaches the main combustion chamber is optimal for the best combustion and it's super hot and in a gaseous form which helps it burn even more efficiently.

The downside is that it's probably twice the complexity and it's startup cycle is also very complex and must be managed precisely.

But it gets the most amount of thrust out of the available fuel of any current engine design today with the least amount of heat stress on the turbines - which you could use for added safety .. OR use to push the turbines even more to crazy pressures and more power. SpaceX does the latter.