It is specific to the Brikka. They put a pressure valve in the column. The coffee must reach a certain pressure before it starts to flow.

You may try whatever amount of water you want... just don't let it burn in place!

There is a subtle balancing act between the quantity of coffee in the basket (how much headspace you keep) and the amount of water ( a ratio of 10:1 with the coffee -- before making the coffee -- yields good results for me).

So ... if you put less water, that means less coffee... which means more empty space in the basket, which modifies the dynamics.

Pro tip: use as little heat as possible to get the water to a gentle boil. Otherwise you might burn the coffee in the basket. Bad.

In short, you will have to experiment...