I've seen colleagues try < 100% work and what would end up happening is that they would work the usual hours, but just get part-pay. There are really no solid checks and balances other than a manager's goodwill when you start working part-time. The only way you can actually curtail hours is if the manager says "ok, s/he is working part-time, so I'll give them fewer responsibilities." Quite often, project deadlines and not-so-nice managers ensure that you end up working full or more hours even if you are officially part-time.