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 (and therefore your FiT) 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. This works well because most generators have the power to "switch off" when their generation isn't needed. Just like the big generators you'll be debited rather than credited for your exports at those times because the grid doesn't need power, and excess generation can cause stability issues. The same applies when your FiT is looking very low - the grid doesn't "need" much more energy at the moment.
The minimum possible wholesale price of energy in Australia is capped at -$1.00/kWh.
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 > $15 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!
As you can see, with Amber you are exposed to both the downsides, and the upsides, of the wholesale market for solar generation. A home battery isn't required to do well with Amber, but it will help you make the most of your solar within the wholesale marketplace.
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.
But you may be better off financially with a traditional relater with the highest FiT if you have a large system, export a lot of solar, and don't have a battery.