A follow-on raises fresh capital. It can be marketed over several days with a mini roadshow, or launched overnight as a bought deal where the bank buys the whole block after the close and re-sells it before the next open. Overnight deals move fast precisely so the market cannot short the stock down into the print.
The share price reaction is usually negative in the short run: supply arrives at a discount and management has signalled it thinks the stock is a reasonable currency at this level.
Example: 120M shares outstanding, stock at $46. The company sells 15M new shares at $43.50. It raises $652M, share count rises 12.5%, and the stock typically settles near the $43.50 print until the new supply is absorbed.
Related: secondary-offering, dilution, offering-discount, shelf-registration, underwriter