How to transfer processing to another back-end?

Question

Nginx

There is nginx and two back-end servers behind it (there may be more in the future). The first back-end performs pre-checks, and if successful, must pass the connection to another back-end to return the result. In the future, there may be several such back-ends for transferring a connection.
If it matters, the connection goes over websockets/tls.
What is the best way to implement this switch via nginx?