When spot rates turn out to be lower (higher) than implied by the forward curve, the forward price will increase (decrease). A trader expecting lower future spot rates (than implied by the current forward rates) would purchase the forward contract to profit from its appreciation. What is the logic behind this statement
Suppose that you buy a 2-year, zero coupon bond (to keep things simple); the 2-year spot rate is 2% and the 1-year spot rate is 1%. The implied 1-year forward rate starting one year from today is 3.0099%. You pay $961.17 (= $1,000 / 1.022) for the bond.
Fast forward to one year from today. If the 1-year spot rate is 3.0099%, then you can sell the bond for $970.78 (= $1,000 / 1.030099), and make a 1% (= ($970.78 − $961.17) / $961.17) profit, as expected. However, if the 1-year spot rate is lower than 3.0099% – say, 2% – then you can sell the bond for $980.39 (= $1,000 / 1.02), and make a profit of _ 2% _ (= ($980.39 − $961.17) / $961.17).
The lower-than-implied spot rate leads to higher-than-expected price appreciation.