I was behind a Callair in an ASW-17 when (I learned later) the tow pilot ran a tank dry. The perception from the glider was that the tow plane had stopped in mid air and the glider was catching up really fast. It doesn't take long for the relative drag of a dirty tow plane with a windmilling prop and a clean glider to eat up the 200' length of the tow rope. And it is not a perception that you normally see in a practice PTT but it sure gets your attention fast IRL. Not knowing what the tow plane's problem was, I left him to sort it out and I connected with the wave and went on my merry way.