otokowa {darekawo mamoru} tokini ichiban chikara ga deru mondaro 漢は誰かを守る時に一番力が出るもんだろ Aman [is at his strongest] when {[he] is protecting someone}, isn't that right?
chikara ga deru 力が出る Power gets out. (literally.) Strength bursts out.
i.e. when a man has someone to protect, he can bring out more strength than otherwise.