What is it called when a poet uses bit from another poem before he starts his poem?

There isn't a specific term for a poet using a bit from another poem *before* starting their own poem. This action wouldn't typically be considered part of the poem itself.

However, there are terms for different ways a poet might reference another poem *within* their own work:

* Allusion: This is a general term for an indirect reference to another work of literature, art, or history. It could be a single word, phrase, or even an entire idea.

* Epigraph: This is a quotation from another work of literature or art that is placed at the beginning of a poem. It often serves to set the tone or theme for the poem, or to provide a contrasting perspective.

* Parody: This is a humorous imitation of another work of literature, art, or music. It often involves exaggerating or changing the original work in some way.

* Intertextuality: This is a broader term that refers to the way in which texts relate to each other. It can involve allusions, parodies, and other forms of referencing.

So, while there isn't a specific term for your example, it might be considered an example of inspiration or intertextuality. The poet is clearly drawing on the previous poem for inspiration, but the actual act of referencing it before the poem starts isn't a poetic technique itself.