Negative, Ghostrider. While they do tense their arms, legs, and abdomen to restrict the blood flow to their lower extremities, they certainly DO NOT hold their breath. In fact, it's more of the opposite. They take quick, short, breaths that sound like "grunts." This helps keep the blood from running from their head. Here's a decent example (listen to his breathing)....
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I saw one of those where a pilot was disqualified for exceeding the manufacturer's G limit. He did 13+ Gs for nearly a whole second. The planes are only certified for up to 12 Gs so they had to ground the plane for a structural inspection.
The crazy thing is that if the planes were certified for higher Gs the pilots would probably push the limit even further.