Dave- if you set up an amortization table on a spreadsheet, you can see from the first payment to the last what each payment's allocation is (how much is principal and how much, interest).
If you have such a spreadsheet, ordinarily the starting balance for a given month's cycle is the ending balance from the previous month; you can, however, include an extra column (extra principal payment) that can be subtracted from the ending balance before being used as the starting blance for the next month. Capeche?
If your P&I payment does not change, then an extra payment will simply serve to accelerate the payoff.
Beware of some shady operators who structure notes such that, if you make an extra payment or such, it us treated as an advance payment of subsequent month(s), wih no credit being given for interest which should never have been allowed to accrue. Not sure how legal that is, but don't specialize in such issues, either. Have seen it where the note was not set up that way, but the "administrators" of the loan processing department just decided to handle it that way, anyway. Pretty cheesy.
If you want, I'll take a stab at building a spreadsheet like I described; I am not a spreadsheet guru, but I imagine even I could pull that one off.
/s/ Spike