These effects include Harmonisers, Vibrato, Pitch benders and octave dividers. Harmonisers in particular should be placed after overdrive. In the opposite order, sending several notes to the overdrive input causes strong inter-modulation distortion where additional, usually low, notes are added. These extra notes may have no relationship to the harmony you intend. Because the quality of most pitch effects is often quite poor, anything that changes pitch without including the original is best placed before overdrive which can help mask the change in pitch affected tone.
When you need to incorporate external devices via MIDI, Helix Floor and Helix Rack processors offer maximum flexibility. In addition to powerful real time MIDI control, they can send up to six separate commands at preset recall, enabling you to, for example, change a preset on a synth, start a sequencer, and turn up a fader on an external digital mixer with a single press of a footswitch. And with their two expression pedal inputs, CV/Expression out, and external amp switching, Helix Floor and Helix Rack processors can serve as the command center for any size rig.