You're asking about how to diffuse the tension after the first act climax.
Technically speaking, that could mean one of two things. Either the first plot point at the end of the first act or the climactic moment, the culmination of the last eighth of the story, also called the climax. (For more info on story structures, see K. M. Weiland.)
The First Plot Point
When it comes to the first plot point, it should act as a point of no return.
For example, Katniss volunteering to be a tribute for District 12 or Gandalf realizing Bilbo's ring is The Lord of the Rings. Katniss cannot undo volunteering and Gandalf cannot, even with lots of ale, forget his discovery. Neither one of them would...
The first plot point usually also represents some form of transportation. This transportation is from "the Normal World" to "the Story World". E.g. in "The Writer's Journey" the first plot point is "Crossing the First Threshold" meaning the hero enters the world of adventure, sets out on the journey, enters the underground, climbs the beanstalk, etc.
If this change or transportation takes the hero from the normal world to the adventure world and in such a way there is no going back, it will naturally result in a moment of having to figure out what just happened, even confusion as to what to do next.
Another thing that can help is the midpoint. In most cases, you don't want the protagonists to start being proactive until you reach the midpoint, but in order to do that, you need a cunning antagonistic force that can keep the protagonists in the dark enough to prevent them from figuring out entirely what is going on until the midpoint.
This suggests that the first plot point could also be confusing, enigmatic, or in some other way transporting the hero not just to an adventure world but also to a strange world, maybe even one with magic and dragons...
What you should not do, however, when you're this early in the story, is to even try to dissipate all the tension... there should always be the orchestral equivalent of at least one quivering violin keeping the reader glued even here.
If you grade the tension of your scenes on a scale from 0 to 10 (10 being the most intense), you should always rework or remove any scene with a tension of 0! No tensionless scenes!
On the other hand, your instinct is right. Good tension does need to vary. Usually, the tension builds until a plot point, then it goes down, but never to 0, and further into the story, the tension might not go down to 1, 2, or even 3 either...
The Climactic Moment
The climactic moment (also "climax" below, even though they are, technically different), should be the biggest moment in your story. It should also be the last scene in your story, except for one or two wrap-up scenes (e.g. showing the new normal to drive home the effect of the climactic moment).
It's definitely "go big and go home" that's the way to go here.
This is the final showdown between the protagonistic and antagonistic forces. This is where your hero definitely defeats the villain or vice versa.
It doesn't have to mean either one of them has to die, just that their plans are thwarted indefinitely; armies routed, plots unveiled, bad guys arrested, etc.
If you feel there's tension and issues left after the climactic moment you can change things like:
- Make the victory/defeat bigger and more permanent
- Raise the stakes
- Of course, prolong the conflict and climax
If you have subplots that need wrapping up after the climactic moment, you can either interleave the subplot climax in the main story climax or try to place the subplot climax before the main story climax.
But even here, you might want to leave some tension to bridge over to book "two" or to just make the reader continue thinking about the story after it's told... (Just don't do a cliffhanger ending, not in the climax... the worst you're allowed to do is to hint at future problems in book two in the wrap-up after the climactic moment...)
For one example of how to do emotional reactions (which may come after at least the very large plot points), you might want to use Dwight Swain's notion of a "Sequel".
In essence, this theory divides the reactive scene/Sequel into three parts:
- Reaction—this would be a kind of reaction that is so strong the character might not be able to do anything else
- Dilemma—this is the part where the character ponders their new situation
- Decision—the result of the "dilemma"-phase is a decision, that will then become the new goal of the character
A lot has been said about the Sequel/Reactive scene/Reaction scene. I might add that I find it helpful to separate it into two types; either it represents a transportation from one scene/time/place to another, or it represents a scene showing a super-strong reaction.
The transport may not even be needed, but if you do need it, keep it short and quick, as few sentences as possible.
The reaction scene, on the other hand, should be written like any scene but the implicit goal of this kind of scene is to react and then figure out a new goal. This scene should be as long as it needs to be.
Also worth adding; of course, your character must react to every stimulus. A golden rule in writing is action followed by reaction, in small, sentence by sentence, as well as larger structures. The reaction type of Sequel is for the large, visceral reactions, for instance, what you would expect from someone having lost a limb.
Of course, all this is a theoretical construct, and apart from not having to have all parts of a Sequel (or one at all), you might also find you'll have to intersperse the parts of one character's sequel or reaction to a big event, with other scenes and events.
Though, your main character, the engine of the story, should probably force the whole thing to stretch out if they need to react over, say several weeks... But then again, if you're hunted by wolves, you may not have time to stop and have lots of reactions either... but of course, reactions add to the story so, even in intense action, some could be used to great effect...