Wave 2 is a corrective wave and should be complete or nearly so. The second impulse wave is actually wave 3 ;)
What triggers these is essentially despair. When everyone willing to sell their holdings out of fear of collapse are finally shorted, the volume will trickle down to nothing. This is often followed by the slight increase and some sideways movement, using time to further frustrate the holders and exhaust their patience. Then when there is no more to be had, the impulses in price happen. Supply and demand takes over, then greed, euphoria. I'll find a chart.