VicS,
I think there is some confusion here as to what you are referring to.
I suspect that when you say "spread", you are referring to trading the difference between two seperate contracts, and not the difference between the bid and offer of a single specific contract.
e.g June Ftse future could be quoted as 4535, whilst the September could be quoted as 4555.
If you think the "spread" or differential between the two contracts should only be 10 points and not 20 points (as quoted in the example above), then you would sell the spread. In that instance you might buy the June at 4535 and sell the September at 4555.
If the spread differential between the 2 months does then narrow to 10 points (June is quoted at 4540, September is quoted at 4550), you will have made a 10 point profit.
i.e you have bought June at 4535, then sold it at 4540
you have sold Sep at 4555, then bought it at 4550.
"Spread" trading and " spread Betting" are 2 completely different things and shouldn't be confused.
The above example would also work if you thought the differential between the 2 contracts might widen. In that case you would buy the spread. i.e sell June, buy September, and hope that the differential would change to anything greater than 20 points.
Not sure if that is clear.