I'm looking at a repair manual for a high powered transistorized audio
amplifier. It repeatedly says:

DO NOT use any test equipment to test or evaluate this amplifier, which
does not have floating grounds.

If the DC voltmeter is AC powered, Float the AC ground wire.

I (sort of) understand what they mean by a "Floating Ground", but I'm
not exactly sure what they mean. And, if I was to use a VTVM which is
plugged into the AC line, or an Oscilloscope (also plugged into an
outlet), how do I achieve a "floating ground"?

Now, lets say I use a pocket battery operated VOM. It's not plugged in,
so there is no ground. Is that safe to use on this device? (Seems to me
that a pocket VOM is NOT actually grounded to earth, so that WOULD be a
floating ground. (I think)....