In both games, TNT has the same style: three (or four in JJ1?) red sticks bound by blue wires, with a kind of stopwatch showing insignificant figures in the middle.
If you “shoot” a TNT, it will be put behind you and stay immobile. It will explode after a few seconds, after an enemy’s arrival or immediately if it is close to Destruct (including Destruct Scenery BOMB), Buttstomp or Collapsing Scenery events (even if the blocks are already destroyed).
TNT does important damage to enemies and blocks; it can even destruct turtle shells and crates (including Scenery Trigger Crate): it will kill every bad guy in one hit; however, efficiency depends on how far you are from your block or enemy.
But it’s hard to hurt players with it; usually, it will only project him to close tiles (it can have the same effect on the player who placed the TNT). You can make your adversaries lose hearts with this weapon by making him be between two. However, if you kill someone with TNT, you should not earn roasts; the player will just die, lose his weapon, regain his hearts and go to a start position without sending any message in the chat. This is fixed by JJ2+.
TNT is usually used by players to destroy blocks, especially in races; it can also be used to slow down a player by making the weapon projecting him back.
Eat your lima beans, Johnny.