so i was looking at the machine curse psychic power. it says that a librarian can target a single vehicle or a number of mechanical devices (such as guns or servitors). does this mean, "hey, you heretics! your guns are all jammed...come fight hand-to-hand with us space marines"? maybe i play to many other rpgs, but are there no, uhh...saving throws for the opponents, or is it just based on passing the librarian's own WP test?

Yup, the psyker only has to pass his WP test to have the weapons jam. The recipients don't have to engage, they can spend a round to make their BS test to clear the jam as per normal. It just gives the marines a little breather to charge with.

How does this power work against tech marines or any marine with implants/bionics/cybernetics?

For cybernetics and tech-priest, I transform it in an opposed WP roll and it only give -5xPR to roll with the affected peace of cybernetic. Depending on the situation it can have other effect, like an agility roll to avoid falling prone if it was cybernetic legs for example.