What does Jiro brush on his sushi? I recently watched the movie "Jiro Dreams of Sushi," and noticed that Jiro was always brushing some sort of liquid onto the sushi, but I can't figure it out.

My hypothesis is that it's either some sort of oil (sesame, olive, etc.) or maybe a vinegar of sorts, but I have no idea.
He brushes soy sauce on it, because he knows how much is sufficient to season each nigiri. Actually not just any soy sauce, but nikiri:


  A good sushi chef adds all the flavors the sushi needs before he hands it to the customer. He mixes his own sauce and uses it behind the sushi bar. This sauce is called nikiri.


you can see him brushing it here:



According to this other site,


  Nikiri is typically made using a mixture of soy sauce, dashi, mirin and sake. The sauce has a subtle complimentary umami sweetness that is designed to enliven and enhance the flavour of the fish with which it is being served.