Study Prey, Flush Out, and the Undead

[Circular Reason]

Okay, so I am new to this game, trying to figure out all the mechanics. I am currently a rogue, and want to focus on a knife throwing build. The throwing knife has bleeding and pierce damage. So I keep hearing about how these 'undead' (haven't even gotten there yet) are 'immune' to bleeding and poison and pierce. My question is this, does Study Prey and Flush Out (skills of the Hunter Mastery), which say (in Anniversary) that they reduce resistance to bleeding and pierce, actually do this for undead? As in, will Study Prey and Flush out make the undead susceptible (some percentage, not fully) to bleeding and pierce? or do these not actually reduce those resistances on undead...? Thanks for the help.

[Sebastian]

undead are not immune to piercing. ghosts specifically absorb a lot of it though, along with physical damage.
you also need to know the difference between absorption and resistance. absorption is (for monsters at least) an innate characteristic that cannot be changed. resistances instead can be lowered with debuffs.
immunity can only come from 100% damage absorption. resistance is capped at 80%.

[Blaatschaapje]

I have a piercing / bleeding build myself (duel wield throwing weapons) and flush out and study prey do help vs undead. I use it all the time else there is no killing undead they are very though without those debuffs.