When Mario kicks a shell while traveling at a high speed he is able to catch up to the shell while in midair and bounce off it.
Basic Assisted Midair
If Mario is being pushed by the autoscroll he is able to throw a shell and then slowly catch up to it in midair. If Mario is also being pushed by a conveyor he is able to immediately bounce off the shell.
Note: This is partially patched on levels created after version ?
Mario is able to delay bouncing off the shell if he kicks the shell forward and then performs an air twirl while falling.
Grinder / Bumper Speed Boost
If Mario throws a shell forward and then immediately hits a Grinder (while he is wearing a shellmet) or a Bumper he is able to slowly catch up to the shell in midair.
This is done by throwing the shell upwards have having momentum in one direction and then immediately kicking it in the opposite direction.
If Mario throws a shell upwards just before he hits a sideways spring he is able to bounce off the shell in midair.
By doing a Yoshi Slow Spit the shell will be close enough for Mario to land on when he ditches Yoshi. This is only possible if Mario hits a ceiling after dismounting Yoshi, or by doing a Flutter Jump when in the New Super Mario Bros. U game style.