Metaphorical application v semantic extension

Geee.. time really flies by in class[which is a good sign], so I'm going to share my question from the last class. Here goes...

Otheguy uses the examples (1) Mami, ¿cómo ese niño sabe a Eric? and (2) Papi, tú me prestas esa pluma y yo te la doy para atrás; please, please, préstamela y yo te la doy para atrás. He states: "The saber of (1) appears where speakers of general Spanish would almost certainly use conocer. It is said to be a semantic extension because, on the model of English know, the meaning of saber has been extended to areas that in unimpacted varieties are covered by conocer. The dar para atrás of (2) appears where speakers of general Spanish would almost certainly use devolver 'to return', and is said to be a loan translation or calque modeled on English give back." In his later examination of para atrás, he says that "Speakers of Spanish in the US could very well have gotten the idea from speakers of English that the concept of "behindness" in space could be applied metaphorically to the temporal notion of repetition." How is it exactly that a metaphorical application of para atrás differs from a semantic extension of saber?