A spear tackle is a dangerous way to stop an opponent. The player is picked up and turned upside down, then dropped on the ground (head first). It is a very dangerous move and can result in neck fracture, spinal cord injury, and even death.
In American football prohibits spear-tackling. The rules haven't always been enforced but recent data showing how often serious injuries occur have started to change the picture. Permanent spinal cord injury causing quadriplegia has brought this problem to the forefront.
In Europe spear tackling is much more common among rugby players. Several Australian rugby players are actually well-known for this tackling technique. The National Rugby League penalizes any player seen lifting another player past the horizontal position. The Australian Football League can suspend a player for a spear tackle.
There is still great debate on when the courts should intervene regarding violence in sports. Some maintain spear tackling is a violent assault that is not part of the game and should not be tolerated. Others claim this move is just careless or an error in judgment.
In American football, driving the player's helmet into his body is a subtle form of spear tackling that isn't always easy to recognize. Right now referres make the call but in the future, the courts may be deciding criminal liability.