Take your own conclusions:
if they announce a strike, which you have to do for it to be legal, provider/government can dictate minimum service of 100% (so making strike void, so to speak).
Also they can recall you to work whenever and if you don't show they can fire you or start a disciplinary process (they can suspend your salary for as long as the investigation is ongoing...btw the investigation is made by the ANSP, so they can take as long as they want. Whilst there you're in limbo, since you can't go to court and fight the decision since you're not fired...blackmail anyone?).
In theory this all is to guarantee the service, which is the excuse they have been using to introduce the royal decree, when there was no imminent threat to it from start (it my have been better or worse, but there was service). Now controllers are 21st century slaves. The only option would be to just not go to work altogether. Let's see how much sympathy that creates amongst the public/coworkers/users/etc.. also they think"if they have done us this for nothing, what would they do then?". Unemployment in Spain is close to 20%, how many controllers can just jump out? (they're looking into going elsewhere, I can give faith to that, I've been asked several times if I've heard of offers outside Spain).
A.