If I understand it correctly, you agree to buy something at a market price at a given time. Then you sell until that point, driving the price down and essentially exiting the trade at the same time.
Matt Levine from Bloomberg explained it.
Here are some excerpt
> One fairly technical explanation that we discussed was the “trade-at-settlement” mechanism. In oil futures, you can do a TAS trade in which you agree, at some point during the day, to buy or sell oil futures at that day’s closing price, plus or minus a few pennies. So at 11 a.m. you can agree “I’ll sell futures at 2:30 today, at whatever the settlement price is then.”
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> Here is one really dumb simple way for that to work. You buy 1,000 futures via TAS during the day. You conclude that a lot of people are selling and no one is buying (except you). You think, well, okay, I have to sell 1,000 futures before 2:30, because at 2:30 I am going to get 1,000 futures at whatever the price is then. So you start selling. You sell 100 futures at $10, and the price goes down. You sell another 100 at $5. You sell another 100 at $0. You sell another 100 at -$5. Et cetera; you keep selling—into very thin liquidity, because there are not a lot of natural buyers—and the price keeps going down. By the time you are done, it is 2:30 and the price is -$37.63. The average price that you got, selling your 1,000 contracts, was, say, -$15: You started selling at +$10 and finished at -$37.63 and averaged your way down. But then at 2:30 you buy 1,000 contracts—the contracts you prearranged to buy using the trade-at-settlement mechanism—for -$37.63. You paid people an average of $15 to take oil off your hands, and people paid you $37.63 to take oil off their hands, and you made an average of $22.63 per barrel moving the oil.
[0] https://www.bloomberg.com/opinion/articles/2020-08-04/some-p...