目录权限'S'是什么意思？(不是小写，而是大写)

我下载了Eclipse，解压后做了一些其他的事情，然后突然发现了这个有趣的行为：

^_^ ~/Downloads > sudo chmod 0000 eclipse/

^_^ ~/Downloads > stat eclipse/
  File: 'eclipse/'
  Size: 4096 Blocks: 8 IO Block: 4096 directory
Device: 801h/2049d Inode: 529725 Links: 9
Access: (2000/d-----S---) Uid: ( 0/ root) Gid: ( 0/ root)
Access: 2012-11-22 19:54:57.752017352 +1100
Modify: 2012-09-20 18:16:26.000000000 +1000
Change: 2012-11-22 20:07:49.354016510 +1100
 Birth: -

^_^ ~/Downloads > sudo chmod 0755 eclipse/

^_^ ~/Downloads > stat eclipse/
  File: 'eclipse/'
  Size: 4096 Blocks: 8 IO Block: 4096 directory
Device: 801h/2049d Inode: 529725 Links: 9
Access: (2755/drwxr-sr-x) Uid: ( 0/ root) Gid: ( 0/ root)
Access: 2012-11-22 19:54:57.752017352 +1100
Modify: 2012-09-20 18:16:26.000000000 +1000
Change: 2012-11-22 20:08:19.042016478 +1100
 Birth: -

‘S'权限对一个目录意味着什么？为什么它不让我把它删除？

谢谢。