Closure of regular languages under "inverse second half"
The white token is a placeholder, which just remembers the original state of the red token (of course, the roles of these two tokens can be switched); this is the state at which $M$ would be after reading $\alpha$. The red token corresponds to the $w$ part of the word, and the blue token to its $\alpha$ part.

When reading a symbol, the red token advances according to the rules of $M$, the white token stays put, and the blue token guesses a symbol and advances according to the rules of $M$ (in effect, it guesses one symbol of $\alpha$).

The machine accepts if the blue and white tokens are at the same state (so we guessed correctly the state at which $M$ is after reading $\alpha$), and the red token is at an accepting state of $M$ (so $\alpha w \in L$).