An interesting observation: buybacks don't change the value of a continuing share at all, unless done at a price well below fair value. Which is pretty rare.
(and it takes a large number of shares at a big discount to fair value to make much measurable difference)

So...if the stock price jumps on the announcement (just a hair under 10% today), the new people buying fall into the two general categories:
* Those who incorrectly think the average buyback increases the value of a stock.
* Those who think the buybacks will be done at prices materially below fair value, pretty much meaning they believed the shares were undervalued before the announcement.

Both categories could be called foolish. The first group are simply wrong, and the second group should have been buying those shares already!
They had the money, and they had an opinion about what a share was worth enough to conclude it was undervalued.

Jim