  Recent scattering experiments in the 3D Kitaev magnet $\beta$-Li$_2$IrO$_3$
have shown that a relatively weak magnetic field along the crystallographic
${\bf b}$-axis drives the system from its incommensurate counter-rotating order
to a correlated paramagnet, with a significant uniform `zigzag' component
superimposing the magnetization along the field. Here it is shown that the
zigzag order is not emerging from its linear coupling to the field (via a
staggered, off-diagonal element of the ${\bf g}$-tensor), but from its
intertwining with the incommensurate order and the longitudinal magnetization.
The emerging picture explains all qualitative experimental findings at zero and
finite fields, including the rapid decline of the incommensurate order with
field and the so-called intensity sum rule. The latter are shown to be
independent signatures of the smallness of the Heisenberg exchange $J$,
compared to the Kitaev coupling $K$ and the off-diagonal anisotropy $\Gamma$.
Remarkably, in the regime of interest, the field $H^\ast$ at which the
incommensurate component vanishes, depends essentially only on $J$, which
allows to extract an estimate of $J\!\simeq\!4K$ from reported measurements of
$H^\ast$. We also comment on recent experiments in pressurized
$\beta$-Li$_2$IrO$_3$ and conclude that $J$ decreases with pressure.
