This strategy is all about encouraging the child to say more by expanding on what they have already said.  This is a great way to get the child to start combining words.  For example, if your child says "dog" you might expand on this by saying "Yes! I see a dog!"  If the child then says "Dog eat."  You might expand again and say "You're right, the dog is eating!"  Again, the main idea behind this strategy is to expand on what the child has said, make it longer.  This is a great way to get them to say more and simultaneously build language.