Enhancer: in the owner's manual

Enhancer: in which products?

Axe-Fx II firmware 5: The new Enhancer uses multi-band techniques for a much more natural effect. Also, the effect is mono-compatible with no phasing problems when summing to mono. The effect both widens stereo signals and “stereoizes” mono signals. Low-cut and High-cut parameters allow control over the region of influence. Note that it is NOT recommended to use the Enhancer if just using one side of a stereo output as phasing effects may be encountered. More information
Axe-Fx II firmware 6.01: Added “Classic” mode to Enhancer block which uses old Haas effect delay-based processing. The Type parameter selects between the new (Modern) and old (Classic) enhancer types.

The Enhancer only works when your OUTPUT signal is stereo. Also, don't use it if you only use one channel (left or right) of a stereo signal.
While a stereo input signal works best, the Enhancer can also "stereoize" an incoming mono signal. Of course, the output signal still has to be stereo.

When summing a stereo signal to mono, the Enhancer in Classic mode can cause phase cancellation. The Modern Mode is mono-compatible to prevent this.
Cliff: "Never put the enhancer in parallel or you'll get phasing problems. That's why there is no bypass mode because it should always be in series." source

Make the Panner effect appear “fuller” and “wider” (stereo only) by adding the Enhancer.

Cliff: "Another way would be to use the Delay block. Set the Mix to 100%, dial in the desired amount of delay and set Balance to full right." source