Its just a threaded set screw in the passage to the transfer slots.
They're something of a crutch. We're adding another restriction to the transfer slot - which itself is restriction.
The second restriction will only be effective if its about the same size as the first. If its too big, its like no restriction and visaversa.
But the slot itself is variable restriction - only portion of the slot below the throttle blade is restricting the fluid into the intake. The portion above the throttle blade is a variable air bleed.
So there are lots of good reasons leave that technique as a last resort.