Milnor - Morse theory - theorem 7.2

Question

I am reading the book "Morse theory" by J. Milnor. In the proof of theorem 7.2 an assertion is done:

Assertion 2: The focal points of $(M, q)$ along any normal line $\ell$ occur in pairs with the same multiplicity.

I understand the proof except the last five lines.

Question: Why do the quadratic forms $Q'$ determine the second fundamental form of $M$ at $q$?

I feel like I am overlook something simple.