The Sandman is an unlockable for the Scout in Team Fortress 2. It is on par with the Bat in terms of damage. The special ability of the Sandman is that you can use it to hit a baseball at enemies to slow them. This slowing effect will be determined by distance. The farther away you are from the enemy, the longer they will stay stunned. The maximum time for a slow to last is about 7 seconds, but a critical hit will do the max slow time and make the target completely immobilized, just like 7 seconds would. The Sandman's downside is that the Scout will lose 15 of his max HP for using it, leaving the Scout with 110 health at most.
It is advised that you should only use the Sandman only if you're used to the -15 HP Penalty. This penalty, combined with the Scout's already low health, will make him an easy kill to many other players, especially Heavies and other Scouts. The baseball can be very helpful in many situations. It can help set them up for easier shots, give nearby teammates an opportunity to kill the slowed enemy, or hold off an Ubercharged opponent. However, Pyros can use the baseball to their advantage, especially if they have the normal Flamethrower, as the Compression Blast can fire the baseball back at you. Keep in mind that the baseball will still slow you if it does hit you or a teammate back. Use this in conjunction with the Flying Guillotine, as it will do critical hits on slowed enemies. This will deal about 150 damage, and with the 15 from the ball, and 60 from bleeding, will do a total of 225 damage, allowing a scout to kill a Soldier with the Battalion's Backup, who will have 220 health. This isn't adding the damage you could do after, as any enemy that isn't overhealed will have a max of 75 health left, letting 1-2 shots from any of your shotguns kill them. The health penalty can make the Scout die from a single direct hit from the Rocket Launcher and Grenade Launcher.