Why I still got nothing when I use boolean mode in full text search?

[Moon's Father]

Maybe full text search still have 50% threshold with boolean mode?
Here is my test.
Any reply is appreciated.
mysql> select * from articles\G
*************************** 1. row ***************************
         id: 1
      title: MySQL Tutorial
    content: DBMS stands for DataBase ...
create_time: 2008-06-20 23:05:50
*************************** 2. row ***************************
         id: 2
      title: How To Use MySQL Wel
    content: After you went through a ...
create_time: 2008-06-20 23:05:50
*************************** 3. row ***************************
         id: 3
      title: Optimizing MySQL
    content: In this tutorial we will show ...
create_time: 2008-06-20 23:05:50
*************************** 4. row ***************************
         id: 4
      title: 1001 MySQL Tricks
    content: 1. Never run mysqld as root. 2. ...
create_time: 2008-06-20 23:05:50
*************************** 5. row ***************************
         id: 5
      title: MySQL vs. YourSQL
    content: In the following database comparison ...
create_time: 2008-06-20 23:05:50
*************************** 6. row ***************************
         id: 6
      title: MySQL Security
    content: When configured properly, MySQL ...
create_time: 2008-06-20 23:05:50
*************************** 7. row ***************************
         id: 7
      title: This is a boy
    content: I love him.
create_time: 2008-06-21 18:49:35
*************************** 8. row ***************************
         id: 9
      title: This is a girl
    content: I love her.
create_time: 2008-06-21 18:55:51
8 rows in set (0.00 sec)
mysql>

mysql> select id,title,content from articles where match (title,content)
against
 ('This*' in boolean mode)\G
Empty set (0.04 sec)
mysql> select id,title,content from articles where match (title,content)
against
 ('+This' in boolean mode)\G
Empty set (0.00 sec)
mysql> select version();
+---------------------+
| version()           |
+---------------------+
| 5.1.25-rc-community |
+---------------------+
1 row in set (0.09 sec)
mysql> show variables like '%ft%';
+--------------------------+----------------+
| Variable_name            | Value          |
+--------------------------+----------------+
| ft_boolean_syntax        | + -><()~*:""&| |
| ft_max_word_len          | 84             |
| ft_min_word_len          | 4              |
| ft_query_expansion_limit | 20             |
| ft_stopword_file         | (built-in)     |
+--------------------------+----------------+
5 rows in set (0.21 sec)
mysql>

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