>[[Etymonline for 'propagation (n.)']](http://www.etymonline.com/index.php?term=propagation&allowed_in_frame=0) ... from *propago* (genitive *propaginis*) "that which propagates, offspring," from ***pro-* "forth"** (see [pro-](http://www.etymonline.com/index.php?term=pro-&allowed_in_frame=0)) + * *pag-*, root of pangere "to fasten" (see [pact](http://www.etymonline.com/index.php?term=pact&allowed_in_frame=0)).
>
>[[Etymonline for 'propagation (n.)']](http://www.etymonline.com/index.php?term=pact&allowed_in_frame=0) ... from **PIE root * *pag-* "fix, join together, unite, make firm"** ...

I heed the Etymological Fallacy. [But what are some right ways of interpreting the etymology, to make it feel reasonable and intuitive?](https://english.stackexchange.com/questions/205571/how-does-be-of-combine-to-mean-possess-give-rise-to#comment485482_205571) 

How did ***pro-* + PIE root  *pag-*** combine to mean the modern definition of 'propagation'?