Not sure about a 'sign' of strength per se, but perhaps a...

Not sure about a 'sign' of strength per se, but perhaps a 'result' of the strength (in the background).
I guess any response that infers continuing strength, would be some form of confirmation.
(these explanations aren't 'official' VSA or Wyckoff explanations, they are how I reconcile what I see on the charts, to myself....and then I try to pass them to others)

cheers