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Tech sink refers to units that cost very little gold relative to tech. Preferably these are also units that are good to buy in almost any situation. Examples of ideal tech sink include Pixie, Gauss Charge, Nitrocybe, and Cluster Bolt. Units such as Xeno Guardian, Manticore, and Frost Brooder can also be used as tech sink.
Having tech sink in a particular color available often enables high-tech legendaries that would not otherwise be viable. For example, in a set with Apollo but no other blue units, you likely don't want to get Apollo because much of the blue will end up being wasted or spent on inefficient things like Steelsplitters. But if there is also Pixie in the set, odds are much higher that Apollo is worth getting because you can spend your extra blue on something cheap while you use red and green tech for most of your purchases.