Speed ramps
Simulator
Shutter angle is a fraction of a frame, so holding the angle while the rate climbs makes every exposure shorter. The light does not care that you were trying to change the speed.
What a ramp costs in light
Changing frame rate mid-shot is how a move glides from normal speed into slow motion. The catch is that shutter angle is a fraction of a frame, so the faster the camera runs, the less light each frame gets.
Variable frame rate
Changing the recording rate while the camera is running. Played back at the delivery rate, the fast part becomes slow motion and the slow part speeds up, without any retiming in post.
Shutter angle is a fraction
A 180° shutter means the sensor is exposed for half of each frame's duration. Double the frame rate and half of a frame is half as long, so each exposure gets one stop darker, every time.
The exposure problem
Ramping 24 to 120 is a jump of about 2.3 stops. Unless something compensates, the shot gets visibly darker as it speeds up, which is usually the first thing people notice when their first ramp comes back.
What can compensate
Opening the iris mid-shot, ramping an electronic ND, or raising ISO. Each has a cost: aperture changes depth of field as it goes, ISO adds noise, and only built-in electronic ND is really invisible.
Why not fix it in the grade
Because the darkest part of the ramp is genuinely underexposed, with the noise that implies. Lifting it afterwards lifts the noise with it. The light has to be dealt with at the time.
Where it is used
Any shot that needs to glide between normal speed and slow motion in one take: a fighter landing a punch, a car passing, a skater taking off. Cutting between two speeds reads very differently from gliding between them.