For those who didn't know her -- or know Slayers in general -- Buffy's behaviour might come across as reckless self-endangerment. She made no attempt to stop his jab (assuming she could soak that hit even if it wasn't pulled), and chose instead to snap her elbow against his side. Just below his rib-cage.

When the jab was pulled, she glanced up to catch his eyes for the half-second. Another smile. Clever, she was telling him. Nice move.