Adding to what RetireJapan is explaining:
Let's imagine that you have a floating rate loan and you pay back JPY100,000/month including both principal (50,000jpy) and interests (50,000jpy).
If the interest rates suddenly surge the 25% rule only guarantees you that your maximum monthly payment will be capped at 125,000Jpy/month.
So your are now paying 125,000Jpy/month, but it says nothing about how much of that is principal repayment and how much are interests. If the rates surged like crazy you might now be paying 100,000Jpy/month of interests and only 25,000Jpy/month of principal.
This means that if you don't increase voluntarily increase your monthly payments it is going to take you much longer to repay your loan.