Tags can occur when a fielder either touches a base in a force out situation, or touches a runner while in possession of the ball. Much like the rules surrounding catches, the ball must be “securely and firmly in the hand or glove” when a tag is applied for an out to take effect.
In the case of a force out situation, the fielder must be in total possession of the ball before the batter or runner reaches the base. For tag-outs on a player, they must be applied before the runner touches the base. In either instances, dropped or bobbled balls will not constitute “secure possession.”
Instant replay has changed the way major league fielders apply a tag to a runner sliding into a base. In the past, umpires would rule a runner safe if he beat the tag. Now, if the fielder holds the tag on the runner and during a pop up slide, over slide, etc., ever leaves contact with the base, the runner can be called out. Read about it in the Insider Report.