It's a Siri feature, press the mic button and say "What did [he/she] say" and it'll rewind a bit, turn subtitles on, and then turn them off right around the point where you asked. I've found it to be a very well implemented feature. If I use it more than three times or so I'll just tell it to "turn subtitles on" for the duration.

Edit: s_weber below beat me to the punch here, I had my draft up too long :)