  We exploit rare observations covering the time before and during a normal
outburst in the deeply-eclipsing SU UMa system IY UMa to study the dramatic
changes in the accretion flow and emission at the onset of outburst. Through
Doppler tomography we study the emission distribution, revealing classic
accretion flow behaviour in quiescence, with the stream-disc impact ionizing
the nearby accretion disc. We observe a delay of hours to a couple of days
between the rise in continuum and the rise in the emission lines at the onset
of the outburst. From line profiles and Doppler maps during normal and
superoutburst we conclude that reprocessing of boundary layer radiation is the
dominant emission line mechanism in outburst, and that the normal outburst
began in the outer disc. The stream-disc impact feature (the `orbital hump') in
the H alpha line flux light curve disappears before the onset of the normal
outburst, and may be an observable signal heralding an impending outburst.
