This is very true, a lot of people tell you to take a breath before speaking and this is actually bad advice for most 'blockers' The reason is that your lungs are always sort of floating around 60-70% capacity. You always have some air in there. And to prove this, there will always be a time where something will spontaneously make you laugh, or you might blurt something out to get your point across, etc etc. But when you take a breath before you talk you raise this to >90% (especially when you're nervous and take big breaths) and for blockers this is counter intuitive, as its already hard enough to get air in and out to create sounds, adding a full chest of air into the mix is not a good idea. Thus, by relaxing your stomach (ab) muscles, you let that breath of air out of your lungs and let them sit at an equilibrium. And if you think about times when you're truly relaxed maybe hanging out with your friends, loved ones, yourself, and can talk freely with no issue. This is most likely the reason.