By playing with the rotation, you can give the appearance of flipping over the stencil by rotating it through “Z” space. It’s a bit imprecise. 
If you need to do precision stenciling, then I suggest starting out with an Alpha, and use the “Make ST” button on the Alpha pallet. Then you can flip the alpha around with the controls on the Alpha pallet to re-apply a precise “flip” of your stencil.
If you’re used to free-handing stencils, then free-hand a mask instead, and use the “Create Alpha” button on the masking tab of the Tools pallet to capture the free-hand as an Alpha.
-K