Found this on Investopedia, a quick read on the poison pill....

Found this on Investopedia, a quick read on the poison pill. Given the recent market action, it may be time to investigate this strategy...

A poison pill is a form of defense tactic utilized by a target company to prevent or discourage attempts of a hostile takeover by an acquirer.

Such plans allow existing shareholders the right to purchase additional shares at a discount, effectively diluting the ownership interest of any new, hostile party.

and...

Poison pills mechanism is aimed at protecting the minority shareholders and to avoid the change of control or company management

Lynas is sitting on tremendous assets and it is a pretty visible target. While I don't see any impending threat it might be nice to have in the back pocket. Obviously a hostile takeover needs majority stock ownership,  I'm not sure what the institutional ownership picture looks like (cj?) but what ever the ownership, the stock has been trading for many years and has become fairly well distributed among many different shareholders. This could make market acquisition a bit more difficult for shares purchased on the market...

https://www.investopedia.com/terms/p/poisonpill.asp