Can someone explain: After the first 60 seconds, what was the reason Chauvin kept his knee on George Floyd's neck?

Not with any logical means of conversation.

If you only look for evidence that will confirm your own prejudice , its no wonder that justice is rarely served in such cases as this .

Can you --or anyone else-- point to any evidence of racism during this arrest ,or to any intent on Chauvin's part to kill Floyd , or point to anything ,that suggests that Chauvin knew he was doing Floyd serious harm .

Perhaps dealing with a large offender , who is resisting arrest ,and who is clearly high on something ---  whilst surrounded by an antagonistic crowd -- might affect your actions and judgement too.

Or are you too arrogant and perfect to admit that ?

It's amazing how many people have perfect hindsight ,when sitting behind their keyboards after the event .

That was the procedure in the training manual, and you know that is not what killed George Floyd. 

To keep the trash from flying away.

Reckless disregard for human life.