Power Transformer Voltage Stress when Isolating

[kg4arn]

 I really cannot find this specific question answered on the web or in the forum.  If I missed it, my apologies and please point me in the right direction.

I understand why one should not isolate the safety ground, or float the oscilloscope with an isolation transformer, for operator safety reasons.  I accept this and I am not challenging it.
And although I believe what TEK says about the power transformer stress, I cannot visualize is how it causes voltage stress on the oscilloscope power transformer.  Would someone be kind enough to explain it?
From TEK: 
https://www.tek.com/document/technical-brief/floating-oscilloscope-measurements-and-operator-protection
 
This is dangerous, not only from the
standpoint of elevated voltages present
on the oscilloscope (a shock hazard
to the operator), but also due to
cumulative stresses on the oscilloscope’s
power transformer insulation.
This stress may not cause immediate
failure, but may lead to future dangerous
failures (a shock and fire hazard),
even after returning the oscilloscope
to properly grounded operation!

[Benta]

Your quote seems to refer to a situation where the 'scope is floated... exactly what should be avoided.

What they are referring to is probably transient conditions if the scope is floated. It's casing and really the whole circuit is suddenly moved to a different potential, which does stress the insulation.

If the 'scope is not floated, there is no additional stress on its power transformer.

[Paul Rose]

They mention two (bad) ways of floating the scope:

disconnecting “signal
common” from earth, either by defeating
the grounding system or using an
isolation transformer

If you just disconnect the grounding pin on the mains lead, but do not use an isolation transformer, then you still may stress the internal power transformer's insulation, because there may be a path from the device under test to the neutral mains wire ( which is grounded back at the breaker box ).

[kg4arn]

Thanks to both of you for the replies.