Using a punch and hammer or a pair of pliers, carefully pinch in both sides of the flopper near the location of the pin. Pinch only very gradually until the flopper binds on the shaft when opening at around 45 degrees. This means the flopper will remain open regardless of which way the shaft is facing.
To improve the chances of your flopper catching on the fish, use a pair of pliers to gently and very lightly turn the tip of the flopper away from the shaft. Be careful as too much bend will alter the trajectory of your shaft and the accuracy of the gun dramatically.
When shooting a bottom sand-dwelling fish, like a flathead, or shooting a fish from above, and your spear shaft will penetrate into the sand, chances are your flopper will fail to open if sand is caught under the edges. Tip: in this instance, leave the shaft in the sand, swim down and secure the fish, rather than pulling your shaft back, expecting your flopper to open, as it may not.