"Flatter handling" means that a vehicle or object is more steady, balanced, and loyal when moving or turning, making it easier to control and maneuver. Full definition
I like the ones with flat handles the best. (dieselcrew.com)
The relatively flat handle of the so called «hockey stick» is most likely a wholly misleading indication of the extent of real world natural variability. (wattsupwiththat.com)
The suspension delivers even tighter body control, with flatter handling and sharper responses. (ausmotive.com)