The Leg Crawl is a Tango figure, but it is used in other choreography, too.
man steps back on his lead foot and leaves his trail leg extended, to
give her something to crawl upon. The woman steps forward and slowly
lifts her trail leg along the man's outer thigh, her toes pointing
down. She might lower and lift again in a stroking fashion, if she has
the time and is feeling flirtatious. The man can provide a lead, some
encouragement, by rotating his upper body a little to the left with a
little right-side stretch. If she is receptive, it is as though he
draws her leg up like a puppet on a string. Especially in Tango, the
action might be performed quite quickly. He might step back on one beat
of music and then he rotate and she sharply raise her leg on the
next—step and "bang."
Strictly speaking, the Leg Crawl
is really just the action of raising her leg along his, and the cue is
often Back to a Leg Crawl or Corte and Leg Crawl.