For example, let's say you
become convinced that a market should be bought, but prices have already had a sizable run-up. In many instances, if the trade is really good, by waiting for a significant reaction before putting on the entire position you are apt to miss the move completely. However, by adopting a scale-in plan-putting on part of the
intended total position at the market and the remainder on a scale-down basis-you assure that you will at least have a partial position if the market keeps on going, without the excessive risk that would be implied by putting on the entire position after a large, uninterrupted advance.