It's a contextual thing, and the difference appears only when translating to English.
Some things, such as a chair, is masculine in German (der Stuhl), but would probably be called "it" in English. So "I pointed at it [the chair]" in English is "Ich zeigte auf ihn [den Stuhl]" in German.
If, however, we were talking of a man (Bob), we would say "I pointed at him" in English, while still using "ihn" in German.
I think the literal English translation would be 'I showed him up'. Showing someone up means to identify / point someone out, though not used very frequently in everyday speech. http://dictionary.cambridge.org/dictionary/english/show-sb-up