Now, as to why a P wave might have a little dip in its middle, well, baseballs generally follow a fairly direct path. At least they don't generally go up and down and the up again (exception: knuckleball on a good day). So to see what's going on with the conduction to make that sort of tracing, you have to imagine the impulse going towards the electrode (upgoing trace), away from it briefly (down) and then turning back towards it again (up again). This will give you that sort of "M" shape you're describing.
Why does this happen?
One of the cool things about myocardial cells (they have several properties that skeletal muscle doesn't) is their ability to share their electricity with their neighbors. (If one skeletal muscle cell is jolted into contracting, its neighbors do not follow its lead.) Normally the impulse goes down specific tracts and as it passes the cells, they depolarize and contract. The effect is a coordinated muscle contraction, sort of like wringing a washcloth, to expel the blood efficiently.
But if that pathway is damaged (infarct or other trauma), the impulse can't travel as efficiently through the myocardial cells. When it reaches the "break in the wire," as it were, it spreads around the damaged area via the myocardial cells, cell to cell. The contraction still happens, but it's a little different than if it were running on its normal coordinated circuit. And because the electrical impulse isn't running in its normal circuit, the electrode that picks up its signal sees an abnormal pattern-- like a "biphasic" (two humps) P.
This also tells you why atrial fibrillation doesn't give you any P waves at all, just a jagged sorta of line. The atrial have become so stretched out that the normal conduction pathway through them is all torn up. You can help visualise this by holding your fingers interleaved and imagining a straight line drawn on the backs of them from right pinky to left pointer. Slide your hands apart, and there is no more pathway. So the electrical impulses are going willy-nilly all over the atria, and all they can do is produce this uncoordinated quivering effect. No good contraction (increased risk of growing clots in there -> anticoagulant therapy) and a messy line where there ought to be P waves. Cool, huh?