What is the special hiring resource?
Well, we go to the grain: the special hiring resource is basically the "emergency button" that companies have when they feel they are playing chueco in a public tender. That is, if you see something weird in the process, Pam! You can put this resource and try to fix the mess before they leave you out of the cast. All this is regulated in the Public Sector Contract Law in Spain, Roll Articles 44 and following, in case you want to read legal.
Now, don't think you can resort to any decision just because yes. The law is very clear: you can only protest (formally, of course) when the decision puts an end to the process, when you are awarded (or not) the contract, or if they decide to back down and cancel everything. You can also give cane if any resolution seriously affects your interests or leaves you out of play early.
The deadline to move file is not eternal: you have 15 business days since the decision is notified. And be careful, in certain cases such as contracts of framework agreement or dynamic purchase systems, you only have 10 days. Nothing to fall asleep in the laurels.
When you present the resource, the whole procedure freezes. Yes, as if you gave pause. No one can move on until the court is pronounced, and that should happen in a maximum of a month. If they are right, the decision is deleted and the process is receded for them to do it well this time.
In a nutshell: the special resource is like the VAR of public tenders. It serves to make the game fair and nobody puts goals with their hand. If you are a company and move on these issues, you better know how this resource works and, if you get lost in such a rule, look for a good specialized legal advisor, which can save you from more than one brown.