Suppose the spot rate of the pound in 30 days is expected to be $1.45. The 30-day forward rate is $1.40. So, speculators might buy pounds 30 days forward at $1.40 and sell them at the spot of $1.45. If a large number of speculators engage in this, large number of forward purchases will push the forward price up until this speculative demand stops.
The forward rate should move toward the market’s general expectation of the future spot rate.