A leg in a spread refers to one side of the trade. In the first example at the beginning of the lesson with coffee, one leg would be the long July position and the other leg would be the short December position. Many spreads are put on simultaneously to catch a price discrepancy. "Legging" a spread refers to buying or selling one side but not buying or selling the other side, and thus not completing the entire transaction. Some traders do this to try and obtain better prices.

An example of legging would be buying the July coffee and waiting to sell the December coffee at higher prices. In effect the trader is now speculating on the spread as well as the absolute price in coffee. If the market sells off, the trader may lose much more than from the original spread position. If the market moves up the trader will be able to put the spread on for more favorable prices. But why not just buy the July outright in the first place? The trader is effectively outright long or short the one position until the other leg is traded. The trader is really speculating on the absolute level of the market by holding one leg of a spread.