I just bought an 09 G37xS, I noticed my car slings back when I let the car ride from 35mph to 25mph. And sometimes when I apply a heavy foot in the accel pedal trying to reach 40 mph, after letting it go it just bulls me back like Im applying brakes...

 

Can't tell exactly from your description, but it just sounds like what a high compression motor does in low gear. It is unclear if you're just experiencing the car's normal downshift, or if perhaps it isn't up shifting promptly. Are you in DS mode when this happens? Try manually selecting a higher gear and see if it doesn't change the braking effect you're feeling.

 

I think you are describing engine braking. Not sure how much it's pulling back for you but it should be normal when decelerating in DS mode.

 

Theres a TSB for harsh downshifting that is common. Possible your car may not have had it done and should fix the problem. I had the same issue.