With Amber, your solar FiT varies in line with the wholesale price of energy - the same price that the big solar generators get.
The wholesale price is driven by supply and demand.
When the wholesale price goes negative this means that the grid is oversupplied with energy vs. demand. The market wholesale price dips into the negative range to discourage generation during these times, and to help keep the grid in balance. The same applies when your FiT is looking very low - the grid doesn't "need" much more energy at the moment, so the price incentivises you to use your solar rather than export, if you can.
The opposite applies during times when the grid is struggling to meet demand. During these times, such as on the hottest days of Summer, your FiT with Amber can increase to > $1 kWh for brief periods. These peak FiT days often coincide with the hottest and sunniest days of the year during daylight savings - peak time for solar generation!
Over the year, most Amber customers with solar - even if occasionally seeing a negative or very low FiT - will save significantly vs. the DMO (or the VDO in Vic), which is the government's reference price for energy.