Placing EQ bands above 20 kHz
While we cannot hear frequencies above approximately 20 kHz, placing an EQ band above 20 kHz can result in a gentler response below 20 kHz than can be achieved only by adjusting the Shape, Q, or Slope. It is often possible to achieve a similar response with a band below 20 kHz, though it takes some fiddling.
For example, compare the shape of these 6 dB/oct High Shelf bands as gain varies:
The higher the band's centre frequency is placed, the more gain must be increased for a similar response in the audible range, effectively making it more difficult to over-do the EQ move, and easier to find the sweet spot.
So in general, it's easier to create a smooth and subtle high frequency Air lift by placing a 6 dB/oct High Shelf band above 20 kHz.
Quick tip - Using a 6 dB/oct High Shelf cut at 40 kHz to subtly reshape the extreme high end can make aggressive leads sound less fatiguing - a little more analog - without losing the brightness they need to cut through the mix.