It won't let you just flip a clip over the Y-Axis. You have to flip a whole video track over the axis.
To do it, select a track or clip, go to 'Tools' up top, scroll down to Video, and select Track Motion. Fiddle around with it till you get your desired results...it's set up a lot like the Pan/Crop tools. You can also modify 3D space by clicking on the Source Alpha button in the top left of the Track Motion window. Select the 3D Source Alpha option and you can then mess with the Z-axis.
I've honestly haven't messed with Track Motion enough to tell you any tricks though.
