"I understand" is a put off for a Japanese businessman. It translates into English as, "I don't have a clue but I want to save face. Go and talk to somebody else."
One day one of the guys I worked with gave me some instructions and I said, "I understand." So he told me again and, foolishly, I repeated, "I understand." So he told me a third time and I said, "Wakari mashite." His eyes lit up, big and wide like a pair of dinner plates, and he sdaid, "You understand?" Well, that's what I was trying to tell him. When I said it in Japanese he believed me.
That happened to me last week. I got to work and found out I wasn't scheduled for that night (I don't usually get Saturday off) so I just went to the movie theatre then went home and lounged around.