Files with the extension .BK1 are 'Virtua Girl' graphics container files. .BK1 files contain the animation (stripper) data used by the Virtua Girl desktop plug-in software and does exactly what it says on the tin i.e. animated strippers dance (strip) on your pc desktop.
Animation .BK1 files appear in the format of: BRITNEY_DANCE01.BK1
(Warning) The links may lead to full nude content - Adults only.