Micro-tension is a combination of a few different things. It could be the change in tone and pace that signals the reader that something is about to happen. They'll want to know what.
It's the unanswered questions that litter the end of your paragraphs, scenes and chapters. Your readers will want to know the answers.
Micro-tension, in other words, is curiosity and desire. You want to leave your readers in a state of needing to know more. If done well, micro-tension will make your story a page-turner.
Here's a small example taken from one of my stories:
I looked at the passenger manifest, twice. Being a sea-faring man, I had my share of superstition. While this didn’t meet any known superstitions I could think of, the fact that Mr. Abernathy was listed on my passenger manifest left me with a sick feeling in my stomach. And that was enough to salt my drinking water.
If I break this down, there are a couple instances found within the paragraph that I would consider examples of micro-tension. Here it is again with some sections colored differently to indicate what I think fit the category.
I looked at the passenger manifest, twice. Being a sea-faring man, I had my share of superstition. While this didn’t meet any known superstitions I could think of, the fact that Mr. Abernathy was listed on my passenger manifest left me with a sick feeling in my stomach. And that was enough to salt my drinking water.
Hopefully, you want to know why Mr. Abernathy vexes the narrator. Why would his inclusion in the voyage be a problem?
I want to take this a step further and show you a short clip. It's from an adaptation of Stephen King's short story, "N." Watch below and then I'll point out some spots that I thought were great uses of micro-tension.
That's how your writing needs to be. Don't end chapters with breaks where your characters are going to sleep, or the reader feels like they too can step away from the action. Leave a question unanswered. It doesn't have to be a world-shattering question, just something that the reader will want answered; something that will make them turn to the next page.