Never breath in when receiving an attack, always breath out! If you receive a good solid strike to the diaphragm you may die from a heart attack. I remember not breathing out at the exact right time (pardon my English) and crapping water for two weeks! There is no messing around with atemi. It is very serious business! It doesn't take much to hurt someone! Did you know that it only takes 5 pounds of pressure to break someones elbow, not very much. I please ask you to ,like you said, mess around, under the supervision of a qualified instructor. Please!