The diagram shows two T-strike arrangements. On the right, white can perform one, on the left it's black's turn. Characteristic of the T-strike in it's basic form is a trivial choice of capture on the victim's part. On the right, black will capture white's moving man, on the right white will capture black's stationary man. In either case it is irrelevant to the outcome.
The T-strike has many applications in every stage of the game. It also has many disguises. It may be the pointe of a deep combination, or a simple tool to prevent an opponent's progress along a particular line. It may be the last straw in an endgame or a first encounter in an opening.