The Bahtinov Aid uses licensed technology developed by Neils Noordhoek - the very same technology built into the famous Bahtinov Grabber. It allows achieving precise focus using a Bahtinov mask making sub-pixel analysis of the spikes.
In order to get accurate calculations you have to enter the pixel size (for DSLRs it is automatically determined), the focal length and aperture of the telescope. Drag the panel over the preview area to frame the star that you want to use. Use the Cross mode to center the star in “Focusing Region”. After that click on Recalc button. You will see two graphical interpretations of the results “Lines” – the usual Bahtinov pattern and “Circles”. To make the focus perfect join the two circles. Also you can see the focus error/distance in pixels. Try to get the lowest possible reading. Usually the atmosphere turbulence is makes the distance to jump all thetime. Empirically is found that when there is good focus, without changing the focuser position, the aid starts to give equal values, but with different signs (for example -0.12 then +0.12). In the bottom of the aid you can see a graph of the values since the last reset (recalc).
In the Canon EOS LiveView mode is recommended to use x5 Zoom to get pixel resolution close to 1:1, in Nikon use zoom 100% or bit less to fit the spikes. This will give more accurate calculation of the focus error. Also for DSLR LiveView you can use a stacking algorithm in order to lower the influence of the atmosphere turbulence which makes the measurements more accurate, so after every focus change wait a second or two. You can control the number of the stacked images with the + / - buttons. When you are focusing with images you can increase the exposure to get more stable readings.
If the star is very bright, the spikes will be very big and in some cases the Bahtinov Aid will not detect them correctly. You have control by several ways:
For both DSLR and CCD/CMOS modes use the Histogram move the black point to make the image darker
If you use EOS camera, lower the ISO and/or the exposure
If you use CCD/CMOS camera, lower the exposure
After focusing is complete it is good to double check the focus using a short exposure.
Here is a screenshot of Bahtinov Aid when Cross mode is activated. Place the cross in the center of the star and click on Recalc to make spikes detection again.
Focusing Region
It is a "transparent" window that shows the image under the Bahtinov Aid and defines what is the source to be used in the Aid
Forces the Bahtinov Aid to make a new spikes detection. Sometimes the aid make false detection. If this happens use the Cross mode, center the star again and click Recalc.
Defines the telescope focal length. If there is value in the Object Calculator it will be automatically used. This values is needed for accurate sub-pixel calculation of focus error.
Defines the pixel size of the camera sensor. If there is value in the Object Calculator it will be automatically used. This values is needed for accurate sub-pixel calculation of focus error.
For DSLR cameras this parameter is populated automatically using the internal database for all Canon EOS and Nikon models.
Defines how many frames from the DSLR LiveView stream to stack. Bigger value lowers more influence of the atmosphere turbulence, but needs more time to build an image and to show the effect from the focus tuning.
The Bahtinov Aid is accessible from 
Related videos:
