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To be in "the zone". To perform with supreme flair and confidence.
"The kid is in rare form tonight."

-Billy in "Purple Rain"
by major_delmac March 31, 2005
To be acting in very particular fashion on a specific occasion, different from the norm.

The form in acting is very "rare"
Pete, you're in rare form tonight bro
by George Allenby January 17, 2011