The API is a simple send one command and get a reply (and close) TCP/IP socket.
The command can be sent in JSON or simple text.

The text to send a pool switch (to pool 2) is:
'switchpool|2'
 or
'{"command":"switchpool","parameter":"2"}'

The documentation of the API is in API-README

There are 3 sample programs that will talk to the API
API.java
api-example.c
api-example.php

The API has a level of IP address security - that groups commands based on:
 non-priviliged: that report information
 priviliged: that change cgminer
Read more about that in API-README and the options required to allow access and priviliged access.