The Stealth (ST) flag allows the player to be invisible to other players on the radar. The only way to locate a player with a Stealth flag is to either see them on a screen, see their shots, hear them jump around, or nullify their flag's effect through the use of the Seer (SE) flag.
With the ability of locating and tracking the movement of tanks on radar being really powerful and commonly utilized, the Stealth flag allows the holder to hide and plan a surprise attack.
While being able to prevent the opponent from locating the holder is a lot in itself, the actual usefulness of the flag comes from the tactics used while in pursuit. To increase the odds of successful assassination, the holder should hide from the opponent while moving closer, only attempting to shoot when in close proximity. This is to prevent giving away one's location and restrict the opponent from performing any reactions that could save them.
A holder of the Stealth flag should jump as least as possible to stay quiet, as well as always look at where the opponent is facing and where they are moving, so they can position themselves accordingly. Planning the route of the opponent is one of the essential factors, as driving to a foreseen point of intersection and awaiting the opponent behind an object before swiftly killing them is one of the most common and effective methods of Stealth assassinations.