A boy in blue shorts and a t-shirt is poised to hit a tennis ball with a racket.
A boy in blue shorts and a t-shirt is about to hit a tennis ball with his racket.
The boy in blue shorts and t-shirt was about the hit the tennis ball with the racket.
Poised to hit a tennis ball is a rephrasing of about to hit a tennis ball.
Poised to and the phrase "about to" reference the same type of action in this instance.