I'm not sure how it works. If you are selecting cash trades which Knife advises, then you have to wait until it clears. If you have enough in your portfolio to cover a margin purchase, then you might be able to buy sooner using margin purchases.
I have Fidelity and if I sell one stock, I can buy another stock the same day using the sales amount (as long as I don't exceed the expected proceeds of course). If I transfer funds from my bank, they allow me to use it to buy shares the next day. (Whether the transfer is complete or not).
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