  During a 2018 outburst, the black hole X-ray binary MAXI J1820+070 was
comprehensively monitored at multiple wavelengths as it underwent a hard to
soft state transition. During this transition a rapid evolution in X-ray timing
properties and a short-lived radio flare were observed, both of which were
linked to the launching of bi-polar, long-lived relativistic ejecta. We provide
detailed analysis of two Very Long Baseline Array observations, using both time
binning and a new dynamic phase centre tracking technique to mitigate the
effects of smearing when observing fast-moving ejecta at high angular
resolution. We identify a second, earlier ejection, with a lower proper motion
of $18.0\pm1.1$ mas day$^{-1}$. This new jet knot was ejected $4\pm1$ hours
before the beginning of the rise of the radio flare, and $2\pm1$ hours before a
switch from type-C to type-B X-ray quasi-periodic oscillations (QPOs). We show
that this jet was ejected over a period of $\sim6$ hours and thus its ejection
was contemporaneous with the QPO transition. Our new technique locates the
original, faster ejection in an observation in which it was previously
undetected. With this detection we revised the fits to the proper motions of
the ejecta and calculated a jet inclination angle of $(64\pm5)^\circ$, and jet
velocities of $0.97_{-0.09}^{+0.03}c$ for the fast-moving ejecta ($\Gamma>2.1$)
and $(0.30\pm0.05)c$ for the newly-identified slow-moving ejection
($\Gamma=1.05\pm0.02$). We show that the approaching slow-moving component is
predominantly responsible for the radio flare, and is likely linked to the
switch from type-C to type-B QPOs, while no definitive signature of ejection
was identified for the fast-moving ejecta.
