Conventional smart styli are self-contained. They all have a controller to detect the applied pressure and the XY location of the pen on the touch screen. Through Bluetooth, this information is sent to the iPad and to be used in an app. This 100% hardware solution leaves very little for the iPad to do. 

The draw back of this design is, it can only be used on supporting apps. (except Apple Pencil) 

SonarPen is a hybrid design. The detection of XY location of the pen is handled by the touch screen, like any dumb stylus. The pressure sensor is read by the audio circuit of the iPad. A piece of code runs on the iPad to combine the XY and pressure readings to provide palm rejection function. 

For SonarPen to work, the iPad needs to do more than one thing (the app and SonarPen) at a time. In our test, even a 2012 released iPad gen 3 is powerful enough to handle the extra SonarPen.

The cool thing is, this hybrid design allows SonarPen to work on apps, even if it is not written to support the stylus.