UMPIRE CALL (4)

This Umpire Call is from one of the Acts on the America's cup. It's included in the TSS sample downloads. The reason why I picket it, was that at first glance I could not understand why rule 15 was used. Here is the diagram:

Blue: America One and Yellow: Prada
What the umpires saw:
At position 2 ,Yellow bore away and gybed onto port tack in front of Blue and was always keeping clear. Blue bore away at position 4 and Yellow double gybed finishing on port tack again at position 5. Blue could have kept clear as windward boat but chose to gybe at position 5 and pass astern of Yellow. Immediately after Blue's gybe there was contact between Blue's spinnaker and Yellow's rigging.
Penalty Blue. Rule 15 .

TSS diagram: UMP_LTW005.TSS
Static image: UMP_LTW005s.gif
Umpire Calls Directory

Can you find out what fact needs to be clear before rule 15 is applicable?