A little history: 

Windows operating system is designed not to read pen pressure when a mouse is used. This is by design as there is no way for the average mouse to register pressure input. When mice came into use way back in the 1980's other was no thought that a pressure reading would be needed, and there were no applications that would be able to accept the pressure reading. So it was just not a thing. 

As all things evolved, the mouse did not so much. They had added features and new looks, but still no pen pressure. 

As that stylus came on to the scene, the users saw a need to be able to have pen pressure, and the software and hardware manufacturers listened, and the stylus with pressure was born. 

Short Story: 

Mouse mode is designed not to have pen pressure. Once you select Mose mode for your Thin or 3-Button pens, you will no longer have pen pressure.

You will also notice that you will need to push the cursor across the screen, just like you would a mouse. 

What does that look like to you? 

The below image shows mouse mode turned on and off. It is easy to see the line quality difference.


You will notice that the writing on the left has no think or thinness to it. It is a solid mass.

The writing to the right has a more adulating quality to it. The line contains both thick and thin lines. This is why most people purchase digital tablets, for the pen pressure effect. 
The panel to the right is the Brush Pallet in Adobe PS.

[1] If you click on Shape Dynamics, you will notice the [2] Delta left of the Control drop-down for Size Jitter.

This is to let you know that it is not detecting pen pressure.

How do you turn mouse mode On or Off?

{1] Select the pen from the left-hand navigation menu you want to work with.

[2] Drop the menu down for Customize, and [3] toggle Mouse Mode On or Off as you like. 
If you like switching between Mouse and Pen Mode, you can often assign that to a button on your Pen or Quick Keys.

Set that up on the bottom button of your pen (and you can do this with any button on your Pens or Pen Tablet.

[1] Select the device you want to set this up on. I selected the 3-Button pen.

[2] Select the button you are going to assign this function to.

[3] Click the drop-down for that button and select Tablet.

[4] Then Select Mouse/Pen Mode Toggle.

Once you have that set, you can now toggle between Mouse and Pen mode with a button press.