In his paper (link), he has the equations:
b1 = k + ƛ - (ρ * σ)
b2 = k + ƛ
k is the rate of mean reversion, ρ is the correlation between the two Wiener processes, σ is vol of vol, what is ƛ?
I have yet to figure out what ƛ is.
It is on page 329 (which is the third page of the article) and represents the market price of volatility risk. I have copied below from the original article:
That is the "price of volatility risk" (see Page 329)
When volatility can change the "attitude" of investors to these changes becomes important for pricing options. This like/dislike for vol increases is captured in the parameter $\lambda$. In practice vol goes up i.e. $dv$ is positive, in bad economic times, such as recessions, when $dC$ is negative. So $\lambda$ is negative.