RT indexes should be declared in sphinx.conf, just as every other index type. Notable differences from the regular, disk-based indexes are that a) data sources are not required and ignored, and b) you should explicitly enumerate all the text fields, not just attributes. Here's an example:

index rt
{
	type = rt
	path = /usr/local/sphinx/data/rt
	rt_field = title
	rt_field = content
	rt_attr_uint = gid
}

RT INDEXES ARE CURRENTLY (AS OF VERSION 1.10-beta) A WORK IN PROGRESS. Therefore, they might lack certain features: for instance, prefix/infix indexing, MVA attributes, etc are not supported yet. There also might be performance and stability issues. However, all the regular indexing features and most of the searching features are already in place, our internal testing passes, and last but not least a number of production instances are already using RT indexes with good results.

RT index can be accessed using MySQL protocol. INSERT, REPLACE, DELETE, and SELECT statements against RT index are supported. For instance, this is an example session with the sample index above:

$ mysql -h 127.0.0.1 -P 9306
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 1
Server version: 1.10-dev (r2153)

Type 'help;' or '\h' for help. Type '\c' to clear the buffer.

mysql> INSERT INTO rt VALUES ( 1, 'first record', 'test one', 123 );
Query OK, 1 row affected (0.05 sec)

mysql> INSERT INTO rt VALUES ( 2, 'second record', 'test two', 234 );
Query OK, 1 row affected (0.00 sec)

mysql> SELECT * FROM rt;
+------+--------+------+
| id   | weight | gid  |
+------+--------+------+
|    1 |      1 |  123 |
|    2 |      1 |  234 |
+------+--------+------+
2 rows in set (0.02 sec)

mysql> SELECT * FROM rt WHERE MATCH('test');
+------+--------+------+
| id   | weight | gid  |
+------+--------+------+
|    1 |   1643 |  123 |
|    2 |   1643 |  234 |
+------+--------+------+
2 rows in set (0.01 sec)

mysql> SELECT * FROM rt WHERE MATCH('@title test');
Empty set (0.00 sec)

Both partial and batch INSERT syntaxes are supported, ie. you can specify a subset of columns, and insert several rows at a time. Deletions are also possible using DELETE statement; the only currently supported syntax is DELETE FROM <index> WHERE id=<id>. REPLACE is also supported, enabling you to implement updates.

mysql> INSERT INTO rt ( id, title ) VALUES ( 3, 'third row' ), ( 4, 'fourth entry' );
Query OK, 2 rows affected (0.01 sec)

mysql> SELECT * FROM rt;
+------+--------+------+
| id   | weight | gid  |
+------+--------+------+
|    1 |      1 |  123 |
|    2 |      1 |  234 |
|    3 |      1 |    0 |
|    4 |      1 |    0 |
+------+--------+------+
4 rows in set (0.00 sec)

mysql> DELETE FROM rt WHERE id=2;
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT * FROM rt WHERE MATCH('test');
+------+--------+------+
| id   | weight | gid  |
+------+--------+------+
|    1 |   1500 |  123 |
+------+--------+------+
1 row in set (0.00 sec)

mysql> INSERT INTO rt VALUES ( 1, 'first record on steroids', 'test one', 123 );
ERROR 1064 (42000): duplicate id '1'

mysql> REPLACE INTO rt VALUES ( 1, 'first record on steroids', 'test one', 123 );
Query OK, 1 row affected (0.01 sec)

mysql> SELECT * FROM rt WHERE MATCH('steroids');
+------+--------+------+
| id   | weight | gid  |
+------+--------+------+
|    1 |   1500 |  123 |
+------+--------+------+
1 row in set (0.01 sec)

Data stored in RT index should survive clean shutdown. When binary logging is enabled, it should also survive crash and/or dirty shutdown, and recover on subsequent startup.