tostop controls whether background jobs are allowed to write to the terminal.
First try this:

stty tostop; (sleep 5; echo hello, world) &

The & causes the command to run as a background job. After five seconds, the
job will attempt to write to the TTY. The TTY driver will suspend it using
SIGTTOU, and your shell will probably report this fact, either immediately, or
when it's about to issue a new prompt to you. Now kill the background job, and
try the following instead:

stty -tostop; (sleep 5; echo hello, world) &

You will get your prompt back, but after five seconds, the background job
transmits hello, world to the terminal, in the middle of whatever you were
typing.

Finally, stty sane will restore your TTY device configuration to something
reasonable.

http://www.linusakesson.net/programming/tty/
