Invert ?

This is phase inversion explained. You will not damage anything using it. Some recordings are out of phase.

 You can hear it. Fortunately, when two signals are badly out of phase, it’s easy to hear. When two similar tracks are mixed, they can sound thinner, tinnier, or harsher, indicating that they have a phase problem.

That invert switch is a nice feature. My gear does not have that so if I wanted to check out the correct phase I would have to reverse the pos and neg on both speakers. I would have to shut off my amp every time and that’s just not worth the inconvenience of doing that!!