  Since Parikh and Wilczek's tunneling method was proposed, there have been
many generalizations, such as its application to massive charged particles'
tunneling and other spacetimes. Moreover, a variant tunneling method was also
recently proposed by Angheben et al that it was independent of coordinates.
However, there are some subtleties in the calculation of Hawking radiation, and
particularly is the so-called factor of 2 problem during calculating the
Hawking temperature. The most popular opinion on this problem is that it is
just a problem of the choice of coordinates. However, following other
treatments we show that we can also consider this problem as a problem that we
do not consider the contribution from P(absorption). Moreover, we also give
some subtleties in the balance method and some comparisons with other
treatments. In addition, as Parikh and Wilczek's original works have showed
that if one takes the tunneling particles' back-reaction into account, the
Hawking radiation would be modified, and this modification is underlying
consistent with the unitary theory, we further find that this modification is
also underlying correlated with the laws of black hole thermodynamics.
Furthermore, we show that this tunneling method may be valid just when the
tunneling process is reversible.
