Tutorial: Purge Valve
Really cool valves which shoot out shots of nitrous into the air. I'll be honest here and say that this is my first attempt at a purge valve, so please bear with me, as I'm learning too! lol..
       
Purge Valve
 
Difficulty - ****
First, hunt for a suitable location for the valve. I'm tempted to go for the scuttle panel on my Digimods MX5! So there.  
Create new layer (Ctrl+Shift+N) and then use the line tool (set on a white colour) and begin drawing lines like on the left. This kinda looks like a spraying action yea? Hmm maybe not..  

Okay.. right.. Smudge tool (R) time! Select quite a large brush set it on 50% and begin smudging out those lines.

Complete..