Let’s look at a simple sentence again.

1. She walks.

Before the words are combined, the verb specifies the subject’s case [nom] and the subject’s 3rd, sing content (orange number 1). If the subject is not compatible, unification is not possible.
When the words are combined, unification takes place. It is nearly unification all the way. Naturally, the content of the VP is identical to the HEAD v. The CATEGORY features are also carried up the tree except for the deletion of the subject from the SUBJ list.

Purple 3, the content of walks contains the content for the subject, orange 1. This content value of the verb is naturally matched up the tree so the content of VP is identical with the content of V and so on. The category features are also matched except that, naturally, the SUBJECT value is deleted at some point once the subject and verb are combined.

I know it’s a bit overwhelming at first, but we’ll practice so don’t worry. Anyway, have a look at the PowerPoint slides here.