To restate FedeForex 's example.
Let's assume 1 MPH = $1.0
You invest 1000 MPH into ETH/USD which goes up 10%.
--> You close your position earning 100 more tokens. Your 1100 MPH is now worth $1100
Now if the price of USDMPH decreases by 10%:
--> Your 1100 MPH is now worth 0.90 x 1100 = $990
On a holistic level you have a net loss of $10.
Now let's consider another scenario that could happen:
You invest 1000 MPH in ETH/USD which falls 10%.
--> Now you have 900 MPH worth $900
You've taken a loss, but if USDMPH increases 10%
--> Your 900 MPH is now worth 900 x 1.10 = $990