[mstibbard]

Stock buybacks are (generally) done by companies when they think their stock is undervalued.

Stock buybacks destroy the quantity of shares that are bought by the company. E.g., if a company had 1,000 shares outstanding and bought back 100 of them, there's now only 900 shares outstanding. This benefits shareholders (assuming they didn't sell into the buyback) because each share is now a larger holding of the company (1/900 > 1/1000).

This decrease in shares outstanding normally corresponds to a share price increase.

I'd speculate that tech companies do buybacks as a way to keep employee remuneration high (via stock) without necessarily needing to pay them a lot of cash.