97 impreza starting problem

I admit any valve overlap should create a 'negative pulse' that could make the test confusing.

And another thing is the bernoulli effect (I think, maybe venturi?) that could make the test a little confusing. To illustrate, find a Post It note or use a strip of paper - even the corner of a full sheet. If you place it very close to your pursed lips while blowing steadily but strongly, there is a point, up close/touching, that has a vacuum feeling. Even pulling a millimeter away, and the paper is blown away, but if you force it up close again, it will appear to stick or suck back TOWARDS the stream of air.

I suspect I would have to acquaint myself to how a normal car performs with a dollar bill or other piece of paper - probably several cars, before I could rely on myself to use the test for diagnosing slipped timing or a bad exhaust valve. And with interference engines, maybe by the time it's VERY obvious the paper is behaving in a way I could feel confident about wrong valve timing, there'd be a lot of noise from bent/broken parts!