When you do a crunch you are lifting your shoulders off the floor. When you do a head lift you are just lifting your head. When you lift your shoulders off the floor it is impossible to hold in your transverse. If you can’t hold in your transverse muscle it will go forward forcefully making the outermost muscles “longer” and the diastasis larger if you have one or creating one if you don’t have one! Also, to strengthen a muscle you must “shorten” it. It is important to strengthen the transverse muscle so it is strong enough to be engaged during bodily functions (i.e. sneezing, coughing, blowing nose, going to the bathroom) and while performing activities of daily living (i.e bending, picking up children or heavy objects).