I'm in a similar situation, and its all setup pre-tax. You elect it at the beginning of the year, and it effectively lowers your salary by the fraction you choose not to work.
Pretty neat, but its capped, otherwise I suspect there would be people who elect to work for 4 months a year (or something similar) for 1/3 of their salary.
Isn't that just regular part time work? People do that all the time.
In the Netherlands there's even a law that states that you can request working part time in basically any fraction of a full work week and the company _must_ grant it unless they can show grave impact to their business that couldn't be resolved by hiring and/or training more people. Your pay is altered in the same proportion, ie you can work 50% of the time for 50% of the pay. This would probably be more difficult in the states, since health insurance is not as strongly linked to having a job over here.
Pretty neat, but its capped, otherwise I suspect there would be people who elect to work for 4 months a year (or something similar) for 1/3 of their salary.