Ah! Finally I got all the pieces: Deckard spits at Roy. Part of the saliva gets in Roy's mouth. His taste is genetically enhanced to the extreme that he can taste DNA. Deck's tastes like his own. Thus, he decides to save his brother and exclaims: Kinship!
I can't make the animated gif of all that, though.
Seriously now, in the 1980 version of the shooting script nothing like the last second catch happens. Here is a transcript of the later vaersion dated February 23, 1981 - scene 126:
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EXT. SECOND ROOF - NIGHT 126
Deckard is hanging there as Batty lands not far from
Deckard's desperate hands.
Batty looks down at Deckard.
Batty grins and takes a seat only a couple of feet
from Deckard.
Deckard's bad hand lets go. He's hanging by one hand.
The street looms way below.
Deckard looks desperately into Batty's cold eyes.
Batty grins and shakes his head at the absurdity of it.
Deckard looks into that awful smile and sees no hope
there.
Batty glances down at his own hand. Spasms again.
Deckard's hand is going. He knows it's over now, he
bites the bullet of his anger. He glares at Batty as
his grip gives way.
DECKARD
Asshole!
Batty meets Deckard's angry eyes.
Deckard's hand continues to slip.
Batty is still looking at Deckard's rage, It moves the
warrior in him, you can see Batty change his opinion.
Too late! Deckard's hand goes.
Batty's hand is like lightning. He catches Deckard's
hand and holds Deckard.
Deckard is suspended above the awesome drop, not sure
why he's not falling. He opens his tightly closed eyes
and looks up.
He looks up into the stern warrior face of Batty, the
cold eyes!
Deckard hangs there and for a moment he has to consider
whether this is the continuation of a cruel game.
The Batty is hauling him up one-handed and with that
scary strength he has.
Deckard is pulled onto the roof where he lies on his
stomach gasping for breath, not moving, just feeling
something solid under him.
Batty looks at the man gasping next to him with the
cold eyes of a man looking at a fish. It is as though
Deckard is some species far below Batty on the evolu-
tionary scale.
Batty's hand cramps again.
Batty looks at it, almost with curiosity.
JUMP CUT TO:
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So, no fuck you, as far as I know, but the writers intended him giving insult in that part. Cool if the alternate takes montage shows another version of this.