It’s a simple mathematical error.
Say that a plane uses a BodyGyro for steering, and you attempt to perform a loop in it.
You tilt up, and up, and up, and the BodyGyro is always slightly ahead of the plane.
Eventually, the plane is tilted 85 degrees up, and the BodyGyro is tilted 95 degrees up.
This is where mathematical problems occur: BodyGyro tilting beyond 90 degrees causes some signs to flip, and as a result instead of the plane rotation and BodyGyro rotation being really close together in one direction, the two are really far apart in another direction. BodyGyro adjusts the rotational velocity in this direction, however the rotational velocity the plane already has allows it to tilt past 90 degrees and thus be mathematically similar to BodyGyro again and the brief mathematical error manifests as a jolt.
This is what I believe happens anyway, based on my experience with this bug in other game engines and that I may see a jolt in rotation at the same time I expect the rotation of BodyGyro to have a sign flip.