Preamble of section C states that Rule 18 does not apply at a starting mark, but Rule 11 (boats on the same tack, overlapped) requires the windward boat (Red) to keep clear of the leeward boat (Blue). Blue is allowed to luff up to head to wind, but no further. Blue's luffing is only restricted by rule 16, i.e. she must change course moderately enough that Red can keep clear.