Black holes in binary systems execute patterns of outburst activity where two characteristic X-ray states are associated with different behaviours observed at radio wavelengths. The hard state is associated with radio emission indicative of a continuously replenished, collimated, relativistic jet, whereas the soft state is rarely associated with radio emission, and never continuously, implying the absence of a quasi-steady jet. Here we report radio observations of the black hole transient MAXI J1820$+$070duringits2018outburst.Astheblackholetransitionedfromthehardtosoftstateweobservedanisolatedradioflare,which,usinghighangularresolutionradioobservations,weconnectwiththelaunchofbi-polarrelativisticejecta.Thisflareoccursastheradioemissionofthecorejetissuppressedbyafactorofover800.Wemonitortheevolutionoftheejectaover200daysandtoamaximumseparationof10$''$, during which period it remains detectable due to in-situ particle acceleration. Using simultaneous radio observations sensitive to different angular scales we calculate an accurate estimate of energy content of the approaching ejection. This energy estimate is far larger than that derived from state transition radio flare, suggesting a systematic underestimate of jet energetics.