I think it's legit.

When you make a normal loan payment each month, you are paying 100% of the interest accrued since the last payment (say 30d worth) and some principle.

If you make an extra payment, say 10d after your normal payment, they are saying that the payment will be applied as paying 10d of interest with the rest going to the principle.

When your next normal payment goes through, you'll pay 30 - 10d = 20d of interest instead of the usual 30d. So more of your normal payment goes to the principle.