Starting your story in the middle of the action rather than at the chronological beginning.
In medias res is Latin for 'in the middle of things,' and it means opening your story at a dramatic or crucial moment rather than building up to it from the start. Instead of explaining who the characters are and how they got here, you drop the reader into a scene that is already happening and fill in the context later. It is one of the oldest storytelling techniques in existence - Homer used it in The Iliad nearly 3,000 years ago.
Your opening is the single most important moment for hooking a reader. Starting in medias res skips the slow wind-up and puts the reader immediately into something interesting. It also creates natural curiosity - the reader has to keep going to understand the situation they have been dropped into. If you are struggling with a flat opening, this technique is often the fix.
The granddaddy of in medias res. Homer begins in the ninth year of the Trojan War, skipping the first eight years entirely. The reader is thrown into Achilles' rage with no warm-up.
The novel opens on the morning of the Reaping, the inciting incident already looming. Collins does not spend chapters explaining the world first - she lets us learn about Panem through Katniss's experience.
The novel opens with a death on stage and the beginning of a pandemic, then moves backward and forward in time, letting the opening scene ripple outward.
Starting in the middle does not mean starting in confusion. Give the reader enough to understand the stakes of the immediate scene, even if they do not know the full picture yet.
An action scene means nothing if the reader does not care about anyone in it. Make sure your opening gives us a character to root for, not just explosions.
The whole point of in medias res is to avoid a slow start. If you follow your dramatic opening with three pages of explanation, you have just delayed the problem.