See here for Bricor's assessment of their FIT rating (Flow Impact Intensity)
Technically, "low-flow" is anything under 2.5 GPM, so 0.5 GPM is pretty astounding. How does it work?
Water enters our patented VACUUM flow "booster" valve where it is aerated and compacted under pressure. Due to the intense force of the vacuum chamber, the aerated water "explodes" as it exits the shower head, creating a powerful shower stream at a very low flow rate (1.25 gallons per minute or less). The powerful release from the vacuum chamber creates a pressure boosting effect that counters the weak flow stream that accompanies low water pressure (from 9 psi on up)
Bricor's showerheads range from 0.5 to 1.25 GPM. From what we hear, Bricor mostly supplies showerheads to hotels and the military, but they do sell to the public, too. To place an order, contact them via e-mail at firstname.lastname@example.org or call them at (830) 624-7228. Retail for $60-$80.