X/Y switching: in the Axe-Fx II owner's manual

X/Y switching: supported by which Fractal Audio products?

Cliff: "The idea behind X/Y is that people often have multiple distortion/chorus/delay/etc. pedals on their board but they rarely have more than one on at a time. So rather than creating more instances of effects, which sucks up CPU, have each instance capable of more than one sound."
X/Y-switching occurs at block level, not on preset level.
The X/Y state of an effect can vary per preset scene.
There's no crossfading when switching between X and Y. If you want crossfading, you can use scene controllers as an alternative to X/Y switching.

Press the "X" key on the keyboard to switch a block between X and Y.
The X/Y-states of the blocks are displayed in the software editor, without needing to select the block first.

X and Y states can be copied using the hardware. To swap them, the software editor must be used.

An assigned modifier always applies to X and Y block states. It's not possible to have different modifier settings for X and Y. The architecture doesn't allow for this. source

Using Axe-Fx II: Recall Effect will always import the entire effect block settings: X and Y. You can't use Recall Effect for X or Y only. Use the software editor instead to copy and paste the X-only or Y-only settings.

X/Y offers an easy way to compare sounds when tweaking. Example:
1. Go to the effect block, state "X".
2. Double-click the "Y" knob on the front panel, to copy the "X" settings to "Y".
3. Select "Y" and change settings. Or doubleclick Bypass to reset the block entirely.
4. Now alternate between X and Y to see and hear the differences.
You can also do this in the software editor.
An alternative approach to comparing sounds, on the Axe-Fx II: save a preset as a Snapshot in Axe-Edit. Select Revert to get the original sound back. Then drag and drop the snapshot file into Axe-Edit.

There's a specific MIDI CC for most blocks which feature X/Y switching. Assign this to a switch alternate between X and Y.
You can configure X/Y effect blocks to engage the effect in either X or Y mode. Example: (source)
1. IA Switch 1, ON message: DELAY 1 ON, DELAY 1 X (CC#47:127, CC#106: 127)
2. IA Switch 1, OFF message: DELAY 1 OFF, DELAY 1 X (CC#47:0, CC#106: 127)
3. IA Switch 2, OFF message: DELAY 1 ON, DELAY 1 Y (CC#47:127, CC#106: 0)
4. IA Switch 2, OFF message: DELAY 1 OFF, DELAY 1 Y (CC#47:0, CC#106: 0)
Or use the controller's group/switch-link functionality.
On the MFC-101 X is green, Y is red. If you prefer the default state to be green, use Y as the default state.

On the FX8 you hold a switch to alternate between the X and Y states. You can set the "hold time" in the global settings.