Why a small amount does so much
Interest is charged on the balance. Every extra dollar you put on the loan comes off the balance today, so it stops being charged interest for every month that's left. Early in a loan that can be twenty-plus years. Nothing else a household can do with a spare hundred dollars pays that reliably, and it's tax-free, because it's interest you don't pay rather than interest you earn.
Weekly, fortnightly or monthly
On a monthly calculation, $100 a week is more money than $100 a month: it's $433 a month. This calculator converts your extra into a monthly amount so you're comparing like with like. What matters is how much goes in each year, not which day it goes in.
The famous fortnightly trick works for the same reason. Half your monthly repayment every fortnight adds up to thirteen monthly repayments a year instead of twelve. It's the extra repayment doing the work, not the fortnights.
Check the extra is actually landing
On a variable loan, extra repayments normally go straight off the balance and can be redrawn later if your loan allows it. Fixed-rate loans often cap extra repayments, commonly somewhere around $10,000 to $30,000 a year, with break costs above that. Look for the balance falling by more than the minimum on your next statement.