Hedge ratio become, reading, secret, between, conversion

RoastBeef

In the secret sauce, I am reading

Hedge ratio = DDp/DDctd * conversion factor for the CTD

This is under the condition that yield spread between the bond being hedged and the CTD issue is assumed to be constanct, the yield beta must equal one.

In which case that yield beta doesn't equal to one?(what's the implication for yield data not become to one)

The formula changes to

Hedge ratio = DDp/DDctd * conversion factor for the CTD * yield beta