the synergy (Atleast in animal studies) is both deca and tren's interaction with igf molecules. nandrolone in rat studies increased igfb3 in all tissue and decreased igfbp4. nand also increases igf-1 but tren is shown to do this to a greater extent (in animals) so with nand decreasing igfbp-4 that means there's less igf-1 binding to this protein and more available to bind to the receptor. igfbp-3 being increased will enhance the half life of igf-1. couple this with the igf-1 increase from tren and you get the synergy....atleast on paper!