Thanks for your detailed explanation!

I understand that for flexi account, whatever amount in excess will be used to knock off the principal amount but for semi-flexi accounts, I understand that we would need to write in or manually request from the bankers to make the adjustments.

Can I write in to the bank only ONCE to inform them that whatever I'm paying additional should be used to knock-off the principal or do I have to write in to them every 2 weeks?

Yes, it is a very sound financial planning to do this. This is why some flexi accounts are charged RM5 to 20 a month, because this facility actually has some value to those who use it.

Say, you keep RM20k as your emergency funding all the time for a year in your flexi-savings account, based on 4.5% annual interest rate, you are saving RM900 from paying interests to the bank this way! Of course, your installment stays the same, but this means your tenure is shortened!

Even if you are not keeping a large sum of money in the flexi account all year, but only in the account for a few days between getting your salary and paying off your commitments, due to the daily rest of most, if not all flexi loan accounts, you are saying quite a bit of interests!
Yes, please clarify with the bank just to be sure. It would be best if you could clarify before signing the loan, but different people have different circumstances!  Good to know you are being proactive against paying unnecessary interests to the banks