Hover bug is...
Hover pad that is placed against wheel.
This give game false info that wheel is on the ground and that disable fly mode activation when tech is airborne. Consequences are that all propellers work like on the ground, so in WASD directions instead all in same time. Second consequence is that SHIFT activate only boost and UP propellers, but not forward propellers so planes are not working with hover bug... but helicopters works much better with hoverbug.
Hoverbug also make force like hover pad is against ground, so if you put enough these combinations your tech will start hovering, and will not stop climbing up in the air. If you balance this very carefully you can make hovering tech that climb very slowly (then you can make it to go down with single propeller) or descend very slowly (and you can make it go up with only one propeller).