One of the ideas why you exhale at the same time, is to bring the abs also in (the correct way to flex abs anyway, but many flex them "out" instead which is incorrect) as this will build the internal pressure and that is also protective for your internal organs. That internal pressure is the same thing you also use when pooping a load (that's why you should keep your mouth shut when sitting in the john, to keep the pressure up). So it may be natural to have that kind of feeling if you're not used to it.
Pavel Tsatsouline explains a lot of this in his weight lifting book "Power to the People", because he instructs people to keep this kind of internal pressure during weight lifting movements too. He actually tells you to concentrate on tightening your sphincther muscle as well (he uses the analogy of trying to keep a coin from falling from between your buttocks). It is hard and funny to try to explain in text like this, but basically, you should think of "inwards, towards the center of your body" for both abs, and sphincter. Pavel also used the analogy of thinking there's a line from your butt-hole to your belly button and the idea is to make this line as short as possible.
Geesh!
