At first stage cutoff the astronauts expected to encounter a single pulse of negative G and the crew would be thrown forward in their straps before the Second stage ignited and recommenced the acceleration. However, they actually encountered a form of pogo which continued for 4 cycles, during which they were “slammed forward, back, forward, back, forward, back, and forward, back. At this point the instrument panel was so blurred the astronauts could not read it.

https://spacerockethistory.com/?p=3313