Reason has three phase effects: the CF-101 Chorus/Flanger, the UN-16 Unison, and the PH-90 Phaser. All three generate various interference patterns by copying an incoming signal, changing its timing or pitch, and then mixing it back with the original signal.
What is phasing?
Phasing gets its name from the concept of identical signals playing back at slightly different times or tunings.
This effect can happen unintentionally if you're not careful. For example, when microphones are placed at different distances from the same source, sound takes longer to travel to one than the other, and the time difference creates interference at the mixing console. When this happens, the sounds are said to be "out of phase" because of timing.
An example of pitch phasing is when two instrumentalists are tuning against each other and a rapid beating cycle, or wave, starts to occur. As the tones get closer in pitch, the beat slows down. These sounds are "out of phase" as well, but the phasing difference is due to the wavelength (pitch) of the sounds, not the timing.
Used deliberately (and with care) timing and pitch phasing can add stereo width to a mono sound, depth to a thin one, or "shimmer" to a voice in a mix. At higher settings drastic filter effects can be created.