When you do this, you're not only taking principal off the back end, you're also increasing the amount of each monthly payment that goes to principal.
Just as an example, if you borrowed $20,000 at 10 percent for a 10 year loan, your monthly payment would be $265. Your first payment would be about $170 in interest and $95 in principal.
If you paid down $5,000 in principal, your monthly payment would stay the same ($265), but the interest component of the first payment would drop to about $125 and the principal component increases by $45. So, now you're paying $125 in interest and $140 in pincipal in your monthly payment. This has a compounding effect -- the principal component in each susbequent payment also increases because of the prior month's increased principal payment.
So, the bottom line is that by paying off 25 percent of your loan, you shorten the loan way more than 25 percent of the duration -- closer to 35 percent. Every extra dollar that you can put to principal has the same compounding effect. Basically, on a 10 year loan at 10 percent, a dollar paid today saves you about 2.2 dollars in the long run. (That's at the beginning of the 10 years -- the further you are into the loan when you pay the extra dollar reduces this number.)