De a tiro (thee-ah-TEE-roh) literally means "of shot". De a tiro typically precedes adjectives. It is used to emphasize a trait of a person or an attitude.
Al tiro (al-TEE-roh) literally means "at shot". Someone is said to be al tiro if that person is alert and ready, or on top of his or her game.
Cautionary note: It is very different to say de a tiro vs. de a tira, which means "police officer style", and the same happens with al tiro vs. al tira, which means "to the cop".
Benito: No me dí tinta de que me estaba echando lámina.
Gualberto: Estás de a tiro bien pendejo.
Benito: I did not realize she was hitting on me.
Gualberto: You're so fucking oblivious.
Pedro: Reinita, necesito que estés al tiro para la siguiente escena.
Penélope: Tu nomás dí rana.
Pedro: Princess, I need you to be ready for the next scene.
Penelope: I'm ready.
Pedro Almodóvar's movies are highly recommendable, but avoid picking up any expression that you hear in them. It can make you sound really silly if you use it in DF.