Depending on how you hit the flipper, it absolutely can add strength to the flip. A good example is a slap-save-like flip, where your hand really nudges the machine simultaneously with the flip. By basic physics principles, some of the momentum is transferred to the flipper and the ball. If for simplicity we treat the playfield somewhat artificially as 2D, the flip/slap is in the x-axis (horizontal), the flipper itself moves on an arc through x/y space and the ball is usually ascending the playfield on a diagonal (x/y), so it squires the x-axis component of any transferred energy/momentum. How much of a difference does this make, by percentage velocity added? I don't know. But having played enough crappy bar pinball setups, I have many times only been able to get a ball up a ramp with a good hard slap!

Depending on how you hit the flipper, it absolutely can add strength to the flip. A good example is a slap-save-like flip, where your hand really nudges the machine simultaneously with the flip. By basic physics principles, some of the momentum is transferred to the flipper and the ball. If for simplicity we treat the playfield somewhat artificially as 2D, the flip/slap is in the x-axis (horizontal), the flipper itself moves on an arc through x/y space and the ball is usually ascending the playfield on a diagonal (x/y), so it acquires the x-axis component of any transferred energy/momentum. How much of a difference does this make, by percentage velocity added? I don't know. But having played enough crappy bar pinball setups, I have many times only been able to get a ball up a ramp with a good hard slap!

You misunderstand.