If you have an options (or derivatives, say) contract and you then trade the physical or spot in order to trigger that contract then you are doing something illegal:
Federal prosecutors in the United States have charged the co-founder of a London-based hedge fund with fraud over an alleged scheme to manipulate the foreign exchange market and trigger a $20 million payment.
Neil Phillips, 52, was arrested in Spain at the request of the US this week. He has been accused of conspiring to artificially manipulate the US dollar-South African rand exchange rate.
He engaged in foreign exchange trades worth hundreds of millions of dollars, according to the prosecutors, who claim that his efforts in December 2017 were designed to drive the dollar-rand rate below 12.50 rand and fraudulently trigger a $20 million payment under a barrier options contract.
I didn’t know that was illegal at all. Humph.