Would/should + infinitive contrasts in syntactic distribution, but not in meaning, with the past tense and the were-subjunctive, both of which express hypothetical meaning in many subordinate clauses. Hence the following cannot be interpreted as unreal conditions [...]:

?If you pressed that button, the engine stopped.
?If you would press that button, the engine would stop.
(A Comprehensive Grammar of the English Language)

The concept is understood as well as the fact that these sentences cannot be interpreted as unreal conditions. What I can't quite work out is the exact meaning of some parts of the original sentence.

I take the 'syntactic distribution' roughly to mean 'position relative to nearby words'. So, the position of would/should + infinitive relative to nearby words is different from the one of the past tense and the were-subjunctive (but at the same time the meaning they convey is the same - "but not in the meaning"). What does it exactly mean? Does it mean that would/should + infinitive occurs in main clauses whereas the past tense and the were-subjunctive in subordinate clauses?

Would you be so kind to help me out with this?

Thanks.