Begin by swinging one poi to your side. Invert (to "buzzsaw" plane) the poi to the inside of your arm. Now switch the poi back to the side of your body ("side" plane), outside of your arm. Try with your other hand. This should be easy. Now spin both poi. Invert one poi at a time. Just like with any move, it takes practice to make perfect. The buzz saw is probably one of the easiest moves, but just like with any move, going from one variation to the next can be the most difficult part.
Spend about 10-20 minutes going from side plane to buzzsaw plane, one poi at a time. Put your poi down, or practice another move for a few minutes, and try practicing the buzzsaw again. This time, try inverting them both at the same time.
I hope my advice is helpful. Keep practicing, and have fun spinning!