Note this cheat is easier to do with UEF.

Start off with two engineers next to a mass building slot, then get one engineer to start building a mass extractor with the same engineer.

After you told it to build it, make it assist the building, then the other engineer will move its arm.

When you see this happen click on it, then tell it to reclaim the mass extractor when it's in the process of being built.

This cheat is useful for rushing or other strategies.