It's like the total life of your mortgage shorten by number of years.
The benefit of lumpsum payment won't see immediately by eye, but if you look deeper, the rough calculation of each payment (say biweekly) is as follows:
Payment = Interest + Principle
Interest = Principle remaining * annual interest / 26
So, if payment stay the same, interest is less (because your principle remaining is lesser due to lump sum payment). Then your Principle portion will increase.
Example (with no lump sum):
Payment 1:
Payment = 150
Interest = 1000 * 5.2% / 26 = 2
Principle = 148
Payment 2:
Payment = 150
Interest = (1000-148) * 5.2% / 26 = 1.7
Principle = 148.3
same example with lumpsum (2%)
Payment 1:
Payment = 150
Interest = 1000 * 5.2% / 26 = 2
Principle = 148
Lumpsum 20
Payment 2:
Payment = 150
Interest = (1000-148-20) * 5.2% / 26 = 1.66
Principle = 148.34
Maybe example is not very huge difference, but you can see the principle PAID after is increasing since interest amount is lesser