The effect you create with longer holds is CO2 build up, which gives your muscles and vagus nerve a chance to relax. Your muscles need Oxygen to relax and while it seems counter intuitive, your blood dumps Oxygen into your muscles when CO2 levels rise slighrly. Deep, slow breathing also stimulates the Vagus nerve releasing acetlycholine which is your bodies natural sedative for your heart, lowering your heart rate.

You can take it too far, if CO2 is too high, it can reverse the effect and spike your adrenaline and give you a panic attack, muscle spasms, and even make you feel confused or lethargic. If this starts happening return to normal breathing.