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

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