When you "put it on knucks" this fist bump lets both you and your friend know that what is being said is the truth, and that its not to be spoken of to anyone else.
Ksystyk: What did you just do?
Kieran: I just hooked up with that cute new girl from math class
Ksystyk: Put it on Knucks
Kieran: boooh yaah!
Ksystyk: You're so lucky dude