X/Y switching

You can set up certain effect blocks to switch between two sets of parameters. For example, with Amp blocks you can have 4 configured amps per preset, of which two can be active. Then use the X/Y knob on the front panel, or an IA switch (toggle) on a MIDI foot controller, to switch between the sets. This essentially doubles the number of those blocks within a preset. X and Y can be the same type of effect, say a RAT drive, but with different parameters. There's no transition between the two. Switching occurs at block level using a MID CC, not on preset level. Source1 Source2 Source3 Source4 Source5 Source6

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." Source

Switch multiple blocks simultaneously by setting the blocks to the same CC.

X/Y switching is nearly instantaneous

X/Y doesn't support delay and reverb spillover. Source

On the MFC-101 X is green, Y is Red. Source

The X and Y buttons also double as user-definable Quick-Jump keys, which can be set to open the EDIT menus of any 2 blocks without going through the grid.

Values 0-63 for the CC select Y, and 64-128 select X. Source

Cliff: "I have implemented an X / Y copy feature in the upcoming firmware. By double-clicking Y you can copy the parameters from the X channel. Likewise double-clicking X allows copying the Y parameters. Copying just the X or Y from another preset is not that easy so that will have to remain a wish-list item." Source