Göran R Buckhorn writes:

On 3 September HTBS posted an entry about Gwen Raverat’s engraving “Boat Race” from 1948. It showed a bump race on the Cam. I am sure most of you readers know what a bump race is and how a ‘bump’ is made, but if you are not really sure, below you will find a short film clip from 1933 showing some eights being bumped at the Lent Bumps on the Cam. By raising his arm the cox in an eight acknowledge that his crew has been bumped and the crew is out of the race, while the crew that made the bump will move up a position for the next day’s race. It’s a wonder that the coxswains are not seriously hurt during these races, though it has happened.

If you are still not sure about the rules, read more here.