This is kind of complicated, so I'll explain with a visual scenario...

Say that there's a ping-pong ball with a needle sticking out of the top of it.

If you were to rotate the ping pong ball 90 degrees along the X or Z axis, then the needle would be horizontal (or parallel to the plane).

Here's where my brain starts to hurt...

If you rotate all three axises at the same time, how then would you calculate when the needle become horizontal, or passes into the southern hemisphere of the pingopong ball?