That's not true. Shocks can control both compression and rebound. Some only do rebound, good ones usually do both.

 

Your referring to compression and rebound,"up stroke", and all shocks work to hold the compression and rebound, otherwise you would bottom out in either direction. I rode enduro and we always had to adjust the rear shock so it wouldn't hit us in the ***, but not hold the inward compression stroke, as to really screw things up. Some auto and truck have both compression and rebound settings. I'm not the best at explaining things, so another input might describe the design of a basic truck or auto shock better. And yes if junk yard shocks look new use them, especially on rear with leaf springs. I'm not that lucky that way in finding good used stuff...
THX for comment

 

Thanks for letting me know.