The etalon filters in the PST and the Solarmax SM40/60/90 front end Ha filters have a bandwidth of about 0.7 Angstrom. This means that the distribution of the light getting through is a Poisson distribution ( bell curve) about 0.7 A across the base. If two filters are used, slightly out of phase (the Tmax adaptor does this) then the combination of the two curves gives a narrow bandwidth of approx 0.5 Angstrom; this gives the improved contrast.
The trick is to be able to do this two stage "tuning" to maximise the contrast. After a few trials, you quickly get the hang of it.
Throughput is reduced by about 20%; not an issue with the Sun.
Colin wrote an article on the subject; explains it much better than me!
http://www.designerinlight.com/solar...ArticlePt1.pdf
Hope this helps.