There's a good write up about it on webBikeWorld here
, and a less technical discussion here
My understanding of the above:
- Snell M2005 did not meet ECE 22-05 standards due to different testing methods, so they couldn't be legally sold in Europe for street use.
- Another hurdle was that for smaller helmets, too much energy was transmitted.
- Snell M2010 is now tested using test methods compatible with ECE 22-05.
- Snell M2010 transmits less energy for smaller helmet sizes than M2005, but doesn't handle "the big bang" as well.
I read the article in Motorcyclist mentioned in the second link, and the general conclusion there (as I recall) was that Snell protected well against the big bang, but in general transmitted more force than other standards for the more typical smaller falls. As a result, it was felt that for more typical impacts, DOT and ECE 22-05 protected better than Snell.
For the most part, if you were comfortable with your M2005 helmet, there's no real incentive to stop using it. And any helmet that meets DOT, ECE 22-05, or Snell M2005 / M2010 is better than nothing.
Disclaimer: I'm not an engineer, and I don't play one on TV. The above is just my opinion.