how does this work glMultMatrixd(P)

It computes the product
M := M * M[sub]f[/sub]
where M is the matrix on the top of the current matrix stack, and M[sub]f[/sub] is the matrix given as argument to the function. Notice that the argument matrix is on the right like any matrix generated by glTranslate, glRotate, glScale, ..., too. In other words: It multiplies the argument matrix with the matrix on the top of the stack and replaces the top matrix with the product.