Unvoiced [j]?

Hi experts!
I am beginning to learn Hungarian and this is what my book says on
page four:
"There is however an unvoiced variant of the voiced fricative (sic)
j in a special phonetic situation: after unvoiced consonants at the
end of a word kapj 'may you have'"

An unvoiced j??? Is the author confused (I mean afaik <j> in
Hungarian is rather an approximant than a sibiland) or does such a
thing exist in Hungarian? What does it sound like? kap + whispering j?

thanks

Harald