the Overhauser effect/method was posposed by Albert W. Overhauser on 1953, that, it can polarizing nuclei. or later called Dynamic Nuclear polarization.

the spin Hamiltonian is

$H = \gamma S\cdot B + \gamma_n I \cdot B + I\cdot A \cdot S$

where S is the electron spin, I is nuclear spin, B is external field strength and A is hyperfine structure coupling tensor. $\gamma$ is a constant such that:

$\gamma = g \frac{e \hbar}{2 m_e}$

it related to the Larmor (angular ) frequency, which is in radius by :

$\omega_0 = \gamma/\hbar$

it is same for proton with a corresponding g-factor and mass, or other nuclei with g-factor and charge and mass.  for example, proton Larmor frequency is:

$\omega_0(p)/ (2 \pi) = 42.5775 MHz T^{-1}$

$\omega_0(e)/ (2 \pi) = - 28.025 GHzT^{-1}$

$\omega_0(n)/ (2 \pi) = 0$

the Hamiltonian commute for both spin eigenstate. thus, we can write the energy as:

$E(m,M) = \gamma B_0 m + \gamma_n B_0 M + A m M$

where m is the eigenvalue of  $S_z$ and M is the eigenvalue for $I_z$. For spin-half electron and nuclei, there will be  4 levels by $m \times M = 4$. the selection rule is:

$\Delta m = \pm1, \Delta M = 0$

this is called the Overhauser Effect.

The Jeffries-Abragam Effect occur when a pumping radiation induced a “forbidden” transition”:

$\Delta m = \pm 1 , \Delta M = \pm 1$