  Extreme sources in the Transient Universe show evidence of relativistic
outflows from intermittent inner engines, such as cosmological gamma-ray
bursts. They probably derive from rotating back holes interacting with
surrounding matter. We show that these interactions are enhanced inversely
proportional to the duty cycle in advection of magnetic flux, as may apply at
high accretion rates. We demonstrate the morphology and ballistic propagation
of relativistic ejecta from burst outflows by numerical simulations in
relativistic magnetohydrodynamics. Applied to stellar mass black holes in
core-collapse of massive stars, it provides a robust explosion mechanism as a
function of total energy output. At breakout, these ejecta may produce a
low-luminosity GRB. A long GRB may ensue from an additional ultra-relativistic
baryon-poor inner jet from a sufficiently long-lived intermittent inner engine.
The simulations demonstrate a complex geometry in mergers of successive ejecta,
whose mixing and shocks provide a pathway to broadband high energy emission
from magnetic reconnection and shocks.
