The flow control knob works very similar to a hose bib or gate valve as its main function is to either fully allow or restrict the amount of water passing through the valve.
The main difference is the flow control limits the stroke of the diaphragm allowing less water to pass. This in turn creates additional pressure loss, allowing you to “fine tune” the performance of your sprinklers. A fully open valve might allow 75+ PSI to pass, which is too much for certain sprinklers like pop up sprays. The flow control would allow you to restrict the flow and pressure to a more manageable level. As you can see in this valve cut away, the flow control literally rests upon the top of the diaphragm to control the stroke of the diaphragm.