We have seen the ECB recently raise interest rates to save the decline of the EUR against the USD. That is the manipulation that you can see as something evident.
The Forex market is an OTC market, that means that the real value of a currency is not always displayed in real time and sudden changes in price can happen when trades that have been happening over the counter get to the public market.
You are talking about a manipulation that is at the Trillion level, the Forex market is supposed to be the only market that cannot be manipulated, but if you can prove that certain banks can cooperate to modify the price then you can prove that everything can be manipulated.