do what he does best 
(ROOT (S (VP (VB do) (SBAR (WHNP (WP what)) (S (NP (PRP he)) (VP (VBZ does) (ADVP (JJS best))))))))
(ROOT (S (VP (VB ) (SBAR (WHNP (WP )) (S (NP (PRP )) (VP (VBZ ) (ADVP (JJS ))))))))