What makes an electron flip to spin-up?

The normal mode of spin for an electron is spin down:with angular momentum L pointing in the opposite direction of motion V (right)but, when a free electron approaches , say, a Helium ion it must , according to Pauli's, flip its spin in the the direction of motion (left: spin up)What makes the electron flip over? who provides the energy for the change? Does the energy required vary form case to case, and if so, what is the formula?...Read more