Effects blending, parallel loops, and phase inversion:
If you want to get more clarity, articulation, and low end from a distortion pedal, one very good way to do that is by mixing it with some of your clean, undistorted signal. There are a few ways to do this:
--Use a parallel effects loop on your amp (if it has one).
--Use an aux loop or a second channel on a mixer.
--Run two amps side by side, one "clean" and one "distorted".
--Use a "blender" pedal.
In all of those cases the clean signal is in parallel (side by side) with the distortion. Your original signal is split in a Y; one leg goes to your effects, and then back out from the effects to rejoin your clean signal on the other side. Here's a graphic representation:
Note that while it's tempting to mix two signals by just passively wiring their outputs together (because of how cheap and easy that would be), the results are usually bad. Passive mixing can lead to loss of volume, loss of tone, loss of lows, or feedback, depending on the specific gear being mixed. So a proper blender is an active (powered, buffered) device, usually using transistors or opamps to do the mixing. In the case of using two separate amps, the "mixer" is actually the ears of the person listening. So all of the finicky issues I'll describe with blending and mixing apply just as much when there is no mixer or blender pedal, because the same things happen in open air, traveling to our ears.
For the majority of people, a blender pedal is the most convenient choice because it is smaller (and often cheaper) than most amps and mixers. A blender combines both the splitting and mixing functions in one box, like this:
If your amp has a parallel effects loop, it works the same way as a blender. Some amp loops have a blend knob, but some are fixed at a 50/50 blend. For the mixing part of a blender, some of them are very simple, just combining the two signal paths equally. Others may provide volume, pan/balance, or EQ controls. And a small number have a "phase invert" switch or other phase control.
Phase is the timing relationship between the waves of two signals. When two similar waves overlap each other perfectly, they are said to be "in phase". When they are mirror-imaged, identical-looking but going in opposite amplitude directions, they are "out of phase". For technically-minded readers: this mirror-imaging is not always caused by changes in timing, so the expression "out of phase" in this context is used to describe the results, not the cause. "Polarity" is a more accurate term for this, but phase is the word that people commonly use.
Two identical signals in phase will result in a higher overall signal level (volume), because the signals add to each other. Two identical signals 180 degrees out of phase will cancel each other out, leaving silence. Most of the time when you mix two audio signals, they will not be 100% identical or perfectly mirrored, so the signal may lose strength or tone but it won't be dead silent. Even such small changes as running the signal through a clean booster or clean EQ can result in changes in the timing or shape of the waves. So the degree that there's any audible problem will depend on the specific devices/instruments and how you have them dialed in. There's no way to predict these interactions in advance, you just have to experiment. Listen carefully for loss of volume, tone, or low end strength.
Many boosts and OD/distortions use an "inverting opamp" to achieve their effect. The inversion of the wave phase is not audible until you mix it with the clean un-inverted signal. So it's easy to assume that any OD/dirt that uses an inverting opamp will "by definition" work best with a blender that has a phase inversion switch. But this assumption doesn't always work out that way, because the wave shape of the effected signal may be different from the wave shape of the original signal, by any amount from a tiny smidge to a total dissimilarity. Additionally, the wave may be slightly out of sync due to lag through the circuit (especially from EQ controls, which incidentally adjust the phase at specific frequency ranges). Since the problem occurs when the two signals are too similar in both shape and timing (but inverted), any audible loss will depend on a large number of small variables, not just the use of an inverting opamp.
So then say you've got a pedal where it is an issue, with audible phase cancellation. If you flip the phase of one of the signals by exactly 180 degrees, is there any guarantee that the two waves won't also interfere then? Nope--and in fact, sometimes that change can make the cancellation worse! It all depends on to what extent the waves happen to overlap in shape and timing. If all we played was sine waves with no distortion, then the inverting opamp assumption would make sense always, and the phase inversion switch would make sense always. But we don't, and they don't.
The very best tool for correcting these dropouts is phase control with more options than just the 0 and 180 degrees of an "invert" switch. Examples with this variable phase feature include the FEA Labs Mixer, Little Labs PIP and LMNOPre, Neve Portico 5016 and 5017, Audient MICO, and Radial Phazer. These offer continuous control between 0 and 180. However this amount of control is not always necessary, just as phase adjustment of any kind is not always necessary. It's a crap shoot, dependent on dozens of tiny variables in your gear and the order of your pedal chain.
For me personally, I find that phase inversion or adjustment is useful with about 50% of the effects I try blending. Of that 50%, most do pretty well with just a simple 180 degree inversion. So for practical low-cost purposes, I think a blender with a phase inversion switch is the best value. Buying a blend without a phase switch can be fine sometimes, but very frustrating other times. And continuously-variable phase control is a luxury that I appreciate, but it is a luxury. The reason I make such a big deal about variable phase in this article is just to help clear up the technical reasons why a phase switch is or isn't effective in different instances. An inversion switch is a helpful tool, but not a magic bullet for all cases.