The rope around the wrists will be loose, but after binding the arms afterwards, it will become impossible for the arms to slip out.
A very simple method of tying that's often mistaken with a vertical knot. Vertical knots can easily be undone, though, so they aren't used in bondage.
A Hon musubi (horizontal) knot should make the rope head parallel to the wrists.
In a vertical knot, the rope around the wrists and the rope head will be in a cross formation.
Why are vertical knots bad? Because if you move on to bind the upper arms and the upper body, and the bound one tries to stretch their arms (when they start to lose their posture or tense up their arms to try and keep their balance), a vertical knot will create pressure on the wrists.
If the knot loosens in a situation like this, the rope might fall off the wrists. Or, even worse, the rope could turn into a noose that squeezes the wrists.
Therefore, it's better to tie the knot itself tightly. The actual rope around the wrists should be a little loose.
You can slide the rope down from the top,
or up from the bottom.
Now, use your other hand to adjust the four portions of rope so that they all have the same amount of tension.
Next, slide your free fingers under the four portions of rope.
Use those fingers to pull the rope through underneath. (Make sure the rope is still loose around the wrists.)
Bend the long end (the end with the rope tails) back 180 degrees, then tie it, and you're finished.
After tying it, check to see if the rope around the wrists still has some slack.
If this person goes on to tie the chest, this rope will cause intense wrist pain.
After the wrists are finished, the rope will be moved around to the chest, so you should move the knot up to the top of the wrists.
If you leave the knot in a lower position like in the picture below, the knot may slide up and scratch against the wrists.
The truth is, even Hon Musubi knots can loosen.
So I recommend this next knot, the Modified Bowline knot (Tomoe Musubi), because it never loosens.
Wrap the rope twice around the wrists.
Behind the rope tail end back, then pull all the rope through, including the bent back part.
Next, slide the rope head through the loop created by the bent rope,
pull the rope tail through, and tighten it.
The great thing about this knot is that pulling on the rope tails doesn't loosen the knot.
(If you pull and loosen the Hon Musubi knot, it will come undone.)
But maintaining equal tension along the rope and keeping it slack at the same time during this knot requires a bit of experience.
So, let me show you an easier and quicker way to tie it, which only requires a little bit of work.
First, wrap the rope around the wrists twice, then pull all the rope through. (So far, it's the same as the Hon Musubi knot.)
Next, bend the rope tail end back, then wrap the rope head around all of the rope, including the bent back part.
After pulling all the rope through, pull the rope head through the loop on the rope tail side that you just made, and lightly tighten it.
Finally, pull the rope tail end to tighten it, and you're done.
If I ever get the chance, I'll also teach you how to do the Bowline knot and the Trek knot(?).