  A two-level system subjected to a high-frequency driving field can exhibit an
effect termed ``coherent destruction of tunneling'', in which the tunneling of
the system is suppressed at certain values of the frequency and strength of the
field. This suppression becomes less effective as the frequency of the driving
field is reduced, and we show here how the detailed form of its fall-off
depends on the phase of the driving, which for certain values can produce small
local maxima (or revivals) in the overall decay. By considering a squarewave
driving field, which has the advantage of being analytically tractable, we show
how this surprising behavior can be interpreted geometrically in terms of
orbits on the Bloch sphere. These results are of general applicability to more
commonly used fields, such as sinusoidal driving, which display a similar
phenomenology.
