By analyzing the public key values alone, the program effectively narrows down the characteristics of the private keys, aiding in identifying whether the private key is positive or negative.
A very simple example: imagine ECC as a clock with hands. After 12 comes 1; if you subtract 5 hours from 3, it would be 10, meaning it will never go negative. This is the concept of a modulo in mathematics. That is, the result will always remain within a defined range, in this case from 1 to 12, and will never yield a negative result. ECC doesn’t work exactly the same, but the concept is very similar when adding and subtracting points.