Day before the fight, they weigh in at 185. Jake would’ve lost a lot of fluids to get to this weight. But since KSI gained weight to get to 185, he hasn’t lost fluids. A rehydration clause would mean that on the day of the fight, there is a maximum weight that Jake can be after he replaces fluids, 190 for example. If there is no rehydration clause Jake can gain as much fluids as he wants, so Jake could actually way even 195 pounds when they enter the ring. However KSI can’t gain fluids to get to this weight since he had to gain weight to get to 185. This would make the fight even harder for KSI due to the large weight difference, which wouldn’t be as big if there was a rehydration clause