Ok, so there was a bit o photoshop involved in overlaying them together.
When you said "timer", i thought you kept the shutter open throughout while you ran back & forth. This is possible only if you keep the ambient light very low and use a strobe for each "exposure". Of course, it's a far-fetched process and entirely unnecessary when you have photoshop.
...sigh.. i miss multiple exposure in film.
Great pic - very nice self-portrait.