He hadn't expected it, but is tall enough that when he jerks back away it splits his lip open, rather than knocking him flat.

He hooks his arm around Sherlock's leg, and brings his own foot down on the other man's groin. He pulls it, at the last second--there's contact but it's not brutal. He wants to see how Sherlock takes to someone holding back on him.

...Well. That, and he's struggling valiantly not to retaliate out of anger.