From M Schwarz's QFT (p 570), Goldstone's theorem indicates that pions are created from the vacuum by the chiral ${\rm SU}(2)$ current $J\_\mu^{5a}$,
$$
\langle 
\Omega|
J\_\mu^{5a}(x)|\pi^b(p)
\rangle
=
ip\_\mu F\_\pi e^{ip\cdot x} \delta^{ab}. \tag{28.30}
$$
We also know that for charged pion decay we can use the 4-Fermi interaction,
$$
\mathcal{L}\_{4F}
=
\frac{G\_F}{\sqrt{2}}
J\_\mu^L J\_\mu^L , \tag{28.31}
$$
where
$$
J\_\mu^L
=
\bar{\psi}\_u\gamma^\mu (1 - \gamma^5) \psi\_d + \bar{\psi}\_{\nu\_\mu}\gamma^\mu\gamma^5 \psi\_\mu
+
\cdots \tag{28.32}
$$
Schwarz proceeds to say that the matrix element for $\pi^+ \to \mu^+\nu\_\mu$ is
$$
\mathcal{M}
(\pi^+ \to \mu^+\nu\_\mu)
=
\frac{G\_F}{\sqrt{2}}
F\_\pi p^\mu \bar{\psi}\_{\nu\_{\mu}}
\gamma^\mu(1 - \gamma^5)\psi\_\mu, \tag{28.33}
$$
as derived from the previous equations.

How is this result obtained? Quite often, I have seen matrix elements written in terms of currents. Is there a way to understand this different approach given an understanding of Feynman rules?