Ekadhikena Purvena
By one more than the previous one (e · ka · dhi · ke · na pur · ve · na)
What it means
Square any number ending in 5 in two seconds. Multiply two numbers whose last digits sum to 10 in one step. The "previous one" is the digit right before the final 5; you add one to it and multiply.
Worked example
Square 65. Look at the digit before the 5 — that's 6. "One more than" 6 is 7. Multiply them: 6 × 7 = 42. Stick 25 on the end: 4225. Done.