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The periodic spectroscopic events in eta Carinae are now well established andoccur near the periastron passage of two massive stars in a very eccentricorbit. Several mechanisms have been proposed to explain the variations ofdifferent spectral features, such as an eclipse by the wind-wind collisionboundary, a shell ejection from the primary star or accretion of its wind ontothe secondary. All of them have problems explaining all the observed phenomena.To better understand the nature of the cyclic events, we performed a densemonitoring of eta Carinae with 5 Southern telescopes during the 2009 lowexcitation event, resulting in a set of data of unprecedented quality andsampling. The intrinsic luminosity of the He II 4686 emission line (L~310 Lsun)just before periastron reveals the presence of a very luminous transient sourceof extreme UV radiation emitted in the wind-wind collision (WWC) region. Clumpsin the primary's wind probably explain the flare-like behavior of both theX-ray and He II 4686 light-curves. After a short-lived minimum, He II 4686emission rises again to a ne

He II 4686 in eta Carinae: collapse of the wind-wind collision region during periastron passage