{poi'i} is a NU abstractor, therefore a poi'i phrase has the form {poi'i [bridi] (kei)}, and behaves as a selbri. Thus it has a syntax different from that of {poi}.
As for its meaning:
• fo'a poi'i (ke'a) broda (kei) = « fo'a is such that [ it/him/he/they brodas ] » = fo'a broda
So it's a kind of syntactic sugar, it helps having more word order freedom. The ke'a within the bridi enclosed by poi'i-kei binds to (stands for) the x1 of the selbri created by poi'i-kei. {poi'i} parallels the English construction "X is such that ... it ..."
Here are a few examples coming from the new-voi page:
• « la .pam. cu melbi gi'e poi'i so'i da nelci (ke'a) (kei) » = « la .pam. cu melbi gi'e se nelci so'i da »
> "Pam is beautiful and is such that many like her." = "Pam is beautiful and is liked by many."
( here, {poi'i so'i da nelci ke'a (kei)}, underlined in the example, is a predicate which means "x1 is such that many things like x1". )
• « la. pit. cu stati gi'e poi'i so'i da manci lo nu ke'a dansu » = « la. pit. cu stati .ije so'i da manci lo nu la .pit. cu dansu »
> "Pete is talented and (is such that) many are amazed by his dancing"
• « mi kecti lo poi'i ke'a jinvi lo du'u ke'a to'e melbi »
> "I feel sorry for the-thing(s)-which is/are-such-that it/they think that it/they are the-opposite-of-beautiful."
> "I feel sorry for those who think that they aren't beautiful."