It consists of adjusting the Black–Scholes theoretical value (BSTV)
by the cost of a portfolio which hedges three main risks
associated to the volatility of the option: the Vega, the Vanna
and the Volga. The Vanna is the sensitivity of the Vega with
respect to a change in the spot FX rate:
.
Similarly, the Volga is the sensitivity
of the Vega with respect to a change of the implied volatility:
.
If we consider a smile volatility term structure with ATM strike , ATM volatility , and where are the 25-Delta
call/put strikes (obtained by solving the equations and where denotes the
Black–Scholes Delta sensitivity) then the hedging portfolio
will be composed of the at-the-money (ATM), risk-reversal (RR) and butterfly (BF)
strategies: