The hydrocracker is what they call an upgrade unit ie it upgrades cheaper products into more expensive ones. Here is a good link as to what hydrocrackers do
http://www.eia.gov/todayinenergy/detail.cfm?id=9650
So in basic terms, when the hydrocraker is not working the refinery has to export fuel oil (cheap) rather than making jet fuel which is in pretty big demand in New Zealand. Hydrocracker needs to have a catalyst replacement every couple of years hence the need for shut downs. It is at the heart of the refinery. Once Catalyst is replaced the amount of thruput they can put thru increases and then gradually reduces until it is due for replacement again (about 2 years from now).
Hope that helps a bit... To get positive margins while the hydrocracker is down is pretty exceptional for NZR.