this is discussed on page 48 of the operations manual. You must restrict the flow otherwise you will starve the pump, but worse, starve the heating element!
http://www.brew-boss.com/v/vspfiles/files/pdf/Brew-BossOperationManualV3.00.pdf
here is the cautionary note:
CAUTION: During mashing, it is important to ensure that the flow of water into the pump
equals the flow out of the pump. This is true for both BIAB and COFI mash methods. The
wort/water level should always be above the temperature probe level. Use the flow regulation
valve on the pump output to reduce flow appropriately. If grains are crushed too finely or
viscous ingredients are used such as rice, oats, or corn, it is possible that the screening on the
filter can become plugged, causing a stuck mash (water can get into the filter but not out). In
this case, it is possible, especially with smaller batches, for the COFI filter to fill with
water/wort such that the supply of water in the kettle outside the COFI filter depletes, both
starving the pump and heater element. This could cause dry firing of the heater element,
causing scorching or worse yet, permanent damage to the element.