Sliders directly adjust goldfinch flight. A chicadee flight path is closer to an undistorted sine wave (steepness ratio of 1). Upswing steepness of 2 approximates a Goldfinch flightpath, oscillating with rapid wingbeats and a steep rise on upswings and gliding on downswings -- a asymmetric, "tilted" sine wave.
On upswings bird flaps wings and rises with steepness ratio set by slider. No flapping on downswings. Upswing steepness (about 2 for goldfinches) distorts the sinewave. X speed determines steepness, while Y position is a sine function. With that steep rise and shallow fall, the sine wave is called "asymmetric" or upwardly "tilted." Chicadee upswing is unaffected because it more closely follows the undistorted sinewave. American Goldfinch song from wikimedia commons.
