  We have observed hysteresis loops and abrupt magnetization steps in the
magnetic molecule V$_6$, where each molecule comprises a pair of identical spin
triangles, in the temperature range 1-5 K for external magnetic fields $B$ with
sweep rates of several Tesla/ms executing a variety of closed cycles. The
hysteresis loops are accurately reproduced using a generalization of the Bloch
equation based on direct one-phonon transitions between the instantaneous
Zeeman-split levels of the ground state (an $S=1/2$ doublet) of each spin
triangle. The magnetization steps occur for $B\approx 0$ and they are explained
in terms of adiabatic Landau-Zener-St\"{u}ckelberg transitions between the
lowest magnetic energy levels as modified by inter-triangle anisotropic
exchange of order 0.4 K.
