From Wikipedia - A spatchcock is a historical term for a culled immature male chicken, but increasingly denotes a preparation technique. The spatchcock, also known as "spattlecock",1 is poultry or game that has been prepared for roasting or grilling by removing the backbone, and sometimes the sternum of the bird and flattening it out before cooking.2 The preparation of a bird in such a manner for cooking is more popularly known as butterflying the bird. The term "spatchcock" is used when the backbone is removed, whether or not the sternum is removed. Removing the sternum allows the bird to be flattened more fully.
In this video I make it easy for you to follow and replicate in your own kitchen.