One of the first things new Pilates clients say is that they don’t get the “breathing”. Understandably so, when in your life have you had someone standing over you telling you when exactly to inhale and exhale. Something the involuntary system of our body takes care of anyway:)
The simple explanation is that exhaling when you exert yourself helps to engage the core muscles of the body helping to stabilize as you mobilize. Think of lifting a heavy box from the floor. If you exhale, engaging your core and then lift from your legs, you have a much better chance of preventing back troubles…then if you just yanked the box up with your back and then herniate a disc…never a good idea.
Our bodies naturally flex slightly as we exhale and extend slightly on the inale…So exercises that incorrporate upper body flexion are usually better executed with an exhale…for example, half curl, and roll up. Exercises that extend the body are usually better executed with an inhale, like in the swan dive and breast stroke.
When in doubt…just breath! Holding your breath is really the only mistake that you can make…