Empress Effects Phaser - Manual (Page 23)

MIDI Clock (aka MIDI Beat Clock)
The Empress Phaser will respond to MIDI Clock messages when it is
in tap and normal mode. MIDI Clock specifies quarter notes,
subdivided into 24 MIDI Messages. Each quarter note corresponds
to 1 period of the phaser’s waveform.
Warning: Changing the speed/ratio knob when MIDI clock is being
sent will confuse the phaser. It will momentarily switch to the knob
setting, and then switch back to the MIDI clock setting causing
general chaos.