I can tell you from my experience that there are two broad styles of harmonizer effects that may require different playing styles. These are delayed harmonies and non-delayed harmonies.
An example of delayed harmonies is PitchFactor Factory preset 10:1, "VaiBallerina". This features a chromatic 6th and a chromatic 4th with different delay times. The idea is that you play in rhythm with these delays to create an illusion that you are in fact playing all of the notes being heard when in reality you are playing only 1/3 of them! For the illusion to work you have to play a new note about at the same time that the second delayed harmony is heard. It's not unlike what U2's the Edge does a lot with regular delay, except in this case the delays are pitch-shifted.
You can hear this in the Eventide PitchFactor demo here
For more examples of this you could try searching YouTube for "Vai Ballerina."
As for preset 1:2, if you play octave notes in the same ryhthm as the delays in that preset then as you rock the expression pedal back and forth you will hear variations in the pitches but with the same rhythm. Again, it comes down to playing in the rhythm of the delays to create the effect.
The other style I mentioned, non-delayed harmonies, can be heard with PitchFactor factory preset 13:1, "Wyld Stallions Solo". This type of effect imitates another guitar player playing the same lead line along with you in harmony. This style effect is best suited to solo-lead parts and does not depend on a particular rhythm.