On my mates injection system there is a throttle position sensor and a rev sensor so say you are on quater throttle and 4000rpm the black box would look up 1/4 and 4000 on a table and this would give a duration for the injector opening time.
The injectors them selves are fed with high pressure fuel and have as many as 12 tiny holes in them to atomise the fuel properly when they are fired.
Hope this helps
Clive W