. the P-ryl group traps the molecule in its unstable enol form. When P-ryl is given to ADP, the enol undergoes a conversion into the more stable ketone, namely pyruvate.
Thus, the high P-ryl transfer potential of PEP comes from the large driving force of the subsequent enol to ketone conversion.
In other words,
PEP is highly energy rich compound (energy rich comp: it should be unstable but the product formed from this should be stable) due to its phosphate-ester bond and therefore, the conversion reaction of PEP to pyruvate coupled by ADP phosphorylation (ATP synthesis). i.e., yields the enol form of pyruvate. It immediately tutomerizes to more stable keto form of pyruvate.