Quoted from Yoko2una:
An ideal pin has the least amount of playfield friction as possible.
So if the ball is moving SDTM, any movement to the machine doesn’t change the path of the ball relative to the floor (it’s still moving in a straight line)... but nudging does move parts of the playfield relative to the ball.
So if you think a right flipper flip can save a ball heading SDTM, you hit the right side of the machine to the left. You’re not moving the ball to the right flipper, you’re moving the right flipper towards the ball.
This is essentially what a slap save is doing.