Underflow Duct

Underflow Duct is a specialized item-transport block that functions as the inverse of Overflow Duct. Its core behavior is to withhold forwarding items under normal conditions and only pass items forward when both of its side connections are blocked. This makes the Underflow Duct a conditional transporter: it does not attempt to push along items unless it detects that both adjacent conduits or inventories are congested.

Underflow Duct has no continuous consumption cost; it does not draw power or expend resources over time to maintain its operation. The block can be upgraded through the Reinforced path, which grants a superspeed effect that is available from both technology branches (described as "superspeed (both techs)" in source materials).

• Function: acts as the opposite of Overflow Duct; forwards items only when both sides are clogged.
• Continuous cost: none — the block has no ongoing consumption.
• Reinforcement: Reinforced upgrade provides a superspeed boost, accessible from both tech trees.

Because its behavior is triggered by side congestion, Underflow Duct is best used where conditional flow control is desired: it can serve as a gate that only allows throughput when parallel lanes are full, or as a complementary component alongside Overflow Duct in complex sorting and buffering networks. The Reinforced superspeed upgrade increases its throughput potential once researched.