Dave,
Have a look here:
http://www.iceinspace.com.au/41-320-0-0-1-0.html
The etalon is the first part of the Ha solar filter system, the second part is called a blocking filter. The etalon on the PST is the adjustable ring just in front of the rectangular body of the scope... I have no idea how it actually works
but it adjusts the frequency of light allowed through the filter.
When adjusted to true Ha wavelength, detail like prominences are easiest to see (no blueshift). To see filaments and detail that is coming out of the sun towards the observer to best effect, the etalon has to be adjusted to allow slightly shorter wavelength through due to the blueshift of the light coming from the filament as it approaches the observer.
Al.