Both forces go through the centre of mass, as the engine doesn't stay firing vertically downwards when the rocket rotates. Your finger on the other hand tries t

Both forces go through the centre of mass, as the engine doesn't stay firing vertically downwards when the rocket rotates. Your finger on the other hand tries to continue pushing upwards as the yardstick rotates about your finger, whcih means the centre of mass of the yardstick moves laterally comapred with the vertical upwards force you apply with your finger. Thus, it creates a moment and rotates.