Navinder Singh Sarao is thought by the Americans to have triggered the 2010 "flash crash" in the US stock markets wiping more than half a trillion dollars off share prices, albeit only for a few minutes. The Americans previously investigated the flash crash and claimed it was just a glitch in the system. Now though, they have fingered "Nav" and want to extradite him.
There is an issue here of legal domain. If Nav broke the law did he break UK law or US law. If he broke US law then that is not an issue because US law does not apply in the UK. If he broke UK law then he should be charged and tried in the UK. Either way there is no substance to a request for extradition to America.