Sawblade is a power-up that rolls across the terrain and, as its name implies, saws players to death. The tricky part about it is that sawblade can kill multiple players and also bounce off walls.
If you start at 4th place and no one has a shield or extra life, use it to kill everyone in front of you.
If you're first and you see someone catching up, find a wall and fire your sawblade, while climbing the wall. The sawblade will bounce off the wall and (hopefully) kill the player.