User blog comment:DancePowderer/Haki Problems/@comment-194.25.108.80-20110121070716/@comment-1595565-20110121165103

Foaming at the mouth happens when just about anyone goes into shock. Also, it was only Haoushoku Haki that did that, Zoro did it with pure strength. Busoshoku Haki hasn't been shown to leave someone foaming at the mouth either.