I found this on the web:

Use the 'Elephant' sequence:
In a sequential order, press <Alt>+<SysRq>+<r> - this will put the keyboard in 'raw' mode. Then, continue the sequence by replacing the <r> above with <s>, <e>, <i>, <u>, <b> - (wait a few seconds between each three-finger combination).
(Mnemonic: Raising Skinny Elephants Is Utterly Boring)

Source: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/418560