I am reading R.E.Gompf and A.I. Stipsicz, 4-Manifolds and Kirby Calculus. I can't understand the 2nd-paragraph of p.101, where they explain framings on the attaching sphere. In particular I cannot understand the sentence "By composing $\varphi$ with a self-diffeomorphism of the second factor of $D^{k}\times D^{n-k}$, we can arrange for [an element of $GL(n-k)$] to be the identity at a preassigned basepoint in $S^{k-1}$." I can't understand why there is such a diffeomorphism. Please explain to me how to solve my problem.
In addition to this,I don't exactly know the definition of "framing." In my understanding this, it is an identification of a normal bundle with the trivial bundle.Is this correct?