[ben1040]

>Hm, but in this concrete example Elon is also a shareholder, so he can buy for 25/share, right?

Nope. The poison pill allows everyone but the one who triggered the provision to buy shares at a discount. From Twitter's press release:

>In the event that the rights become exercisable due to the triggering ownership threshold being crossed, each right will entitle its holder (other than the person, entity or group triggering the Rights Plan, whose rights will become void and will not be exercisable) to purchase, at the then-current exercise price, additional shares of common stock having a then-current market value of twice the exercise price of the right.