ATDTDA (17): "atole con el dedo" (480.36)

[Tim Strzechowski]

"Sure, Lake," the boys holding out their fingers Mexican style as if to say _atole con el dedo_, "but Ma could've heard that anywhere.  She knows you believe everythin she says" (p. 480).



Translates into "to lead someone on," or "to thow a bone" to someone.
 
Within the context of the passage, how does this phrase enhance our understanding of the characters involved?  Why does Pynchon put this particular phrase in Spanish?
 
I'm always curious about why authors who use foreign phrases within their work use *that* particular phrase in a language other than English.
 
Why that phrase here??

And what exactly does it mean to hold out one's fingers Mexican style??  Is that a Ghettospeak phrase out of hip/hop culture?