This tool is to take a preview exposure for focusing or centering purpose.

Set the exposure time and the binning to use for the preview.
You can also change the gain or the “ISO” if supported and activated in the options

This setting is totally independent of the Capture setting.

Click Preview for a single exposure or Loop to take exposure continuously until you stop it with the same button. The light change to green when the loop is active.

If you activate the “preview stacking” in the preferences you also see a Stack check-box.
When it is checked, every preview frame is added to the previous ones.
Use the menu File → Dark frame to define the dark frame to subtract from each image before the addition.
If a bright enough star is present the frames will be aligned on this star.

After you start the Loop, you see the object image to appear progressively in the screen as the number of frame increase. This function is often use in public demonstration.
You can use the menu File/ Save FITS file... to record the stacked image after you stop the Loop.

A new stack is started when you start the Loop, or after you take a preview without this option.