I have problems executing the double thumb
upstroke.. Seems like I'm using too little
force in pull to have that pop-alike sound, but if I use more strength, my nail hurts plenty after couple of those.

Any idea what I might do wrong?

i was some what woindering how to do it myself but i dont like how i cant pluck efficiently after a down and up
+
it not so much a slap and pluck with the thumb but like a flick of the wrist that results in the STOKE of the string down than up again try practicing on thew highest string so the other strings dont interfere