Fallback rise is a chart pattern in which your temperature rises after ovulation but then falls back down immediately. This usually happens at 2-3 dpo. It then goes back up shortly after.
Before O, estrogen levels peak and then drop, causing LH level to peak and then drop, causing ovulation. The theory of fallback rise is that the estrogen level is still dropping after the LH level has dropped, and the progesterone levels have not risen high enough to push the temperatre up. Estrogen causes low temps, progesterone high temps. By the next day, the estrogen level is down and the progesterone level pushes the temperature back up, until youve had a few more high temps after it can be difficult to tell from your chart, your temps are up and down a lot anyway so it is diffult to know, cover all your bases and get lots of bd in just incase!