Depends somewhat on the individual machine, moreso with toploaders. Some of them run a test routine immediately at start of the cycle involving spinning and/or oscillation to judge the weight of the dry load. One reason for the slower flow rates and/or filling in "spurts" is to give the load time to absorb water and the machine time to sense/react to the saturation.
Any machine with an "Add Garment" indicator surely can take more items while the light is illuminated ... but it expects the cycle to be paused and restarted so the controller "knows" there was an interruption that may have been for reason of adding an item.