How to use the action 
3. Create an empty layer named Brush (with capital "B") and inside it brush over the background image the areas you are interested upon using a soft or hard round brush. You don't need to brush the entire subject, thin lines will do best.  You can brush multiple separated lines so your subject will be dispersed into a few areas.
4. Run Hypnotize - Ink in water action from the action panel.
The action will run for a few seconds and generate a folder structure, preserving your original image.
Note: The action generates unique results every time; to generate a new effect just run the action again - a new effect folder will be generated over the previous one in the layers panel.
Note: The way the result look is also influenced by the size of your original image. Images should be at least 1500 px wide in most cases. Larger images will tend to produce "heavy" results.
Note: You might want to increase the canvas size before running the action - generated ink effects might otherwise exit the canvas and be cut by it.

Customizing the effect 

The action produces a folder (layers group) called Hypnotize - Ink in water. Inside this group you will find the main object group, the shadow and a white background layer.
The shadow layer has a motion blur smart filter applied that you can customize to change the length and aspect of the shadow.
The white color fill layer can be changed with any other color - I suggest you keep it white or a very pale color - it's how the action is intended to be.
The main object group contains all the layers of the effect
The 3 general color filters will let you adjust various color looks - contrast, saturation, dark tones.
The Margin effect, if on, is responsible for the blue-pink-magenta color cast around parts of your subject.
The Margin color changer can toggle these colors by changing hue. If you disable the margin effect, a gold-yellow color will be present over your subject. This gold color can be switched off or changed from the Subject Color Filter.
The Top Ink Fill layers are responsible for the ink around your subject. You can toggle on both of them for a more intense look. If your image has a white background around your subject, you can toggle both off for a great dispersion-like effect. If one is off, you can adjust
the other one's blending options - Inner glow to change the color of the ink effect's outer part.
The Subject with inner glow layer can be kept on for a more pronounced ink dispersion effect over your subject.
By editing the blending options, the effect can be customized in multiple ways.