This phrase is known to the vast majority of offline casino dealers. It is common in many countries.
Internal Hand – the dealer's hand on the roulette wheel, located on the side of the drum.
With this hand, the croupier launches the ball. Usually, the dealer sends it alternately right and left on the track.
Also, the croupier's inner hand performs other operations: removes chips from the floor, corrects bets, makes payments and so on.