He doesn't understand how it works, or was explaining it incorrectly.
Your payment stays the same, but essentially you jump "ahead' on your amortization schedule. Paying 10k now versus a year from now will save you money, yes.
See attached excel spreadsheet (best one I've found for putting in additional payments).
Input your own: Loan Amount, Annual Interest Rate, Term of Loan in Years, First Payment Date
In the provided example (100k loan, 30 years at 3.75% started last June) if you pay 10k in a week (Feb 2013, Cell D33) it shows interest savings (in the summary, above the amortization table) of 17,276. If you wait until Feb 2014 to make that payment, you only save 16,357, a difference of about $900.