Here you go Samoth. It's an application of Ito's Lemma, which is one of the fundamental assumptions of advanced financial derivatives, which relies heavily on stochastic calculus, and financial engineering.
Non-continuous semimartingales
Itō's lemma can also be applied to general
d-dimensional
semimartingales, which need not be continuous. In general, a semimartingale is a
cadlag process, and an additional term needs to be added to the formula to ensure that the jumps of the process are correctly given by Itō's lemma. For any cadlag process
Yt, the left limit in
t is denoted by
Yt-, which is a left-continuous process. The jumps are written as Δ
Yt =
Yt -
Yt-. Then, Itō's lemma states that if
X = (
X1,
X2,…,
Xd) is a
d-dimensional semimartingale and
f is a twice continuously differentiable real valued function on
Rd then
f(
X) is a semimartingale, and
This differs from the formula for continuous semimartingales by the additional term summing over the jumps of
X, which ensures that the jump of the right hand side at time
t is Δ
f(
Xt).