Recent scattering experiments in the 3D Kitaev magnet β-Li2IrO3 have shown that a relatively weak magnetic field along the crystallographic b axis drives the system from its incommensurate counter-rotating order to a correlated magnet, 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 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 Γ. Remarkably, in the regime of interest, the field H∗ at which the incommensurate component vanishes, depends essentially only on J, which allows us to extract an estimate of J≃4 K from reported measurements of H∗. We also comment on recent experiments in pressurized β-Li2IrO3 and conclude that J decreases with pressure.