Hi there.

ExoSAP contains Exonuclease 1 and Shrimp Alkaline Phosphatase (both hydrolytic enzymes) as the active reagent to breakdown non-PCR product (primers and excess dNTPs). The Exonuclease will degreade anything single stranded (such as single strands of DNA, and residual primers but not your dsDNA PCR product), and the Shrimp Phosphatase will hydrolyse unused dNTPs. I don't know the specific biochem, but that's the overview. 

Page 4 of the ExoSAP-IT protocol titled 'overview' has an explanation of how this occurs (see the link below).

Hope this helps.

https://www.fimm.fi/sites/default/files/SeqLab_Exosap_USB.pdf

This pitcure clearly explain the mechanism of Exosap