This question have been put on hold (the question itself is not relevant here).
What is the mechanism behind the on-hold lock?
As I understand, any user with the "protect questions" privilege can put question on hold, but certain number of votes by unprivileged users can achieve the same result.
How many votes are needed? What are the fine details of voting for close? Do votes expire?
"on hold" is really just "closed" but renamed to encourage users to edit and improve the question to get it reopened again.
After a while the "on hold" tag gets replaced by "closed"
Any normal user can vote to put a question on hold (I think there may be a rep threshold but can't remember off-hand). I think it takes 6 votes from normal users for a question to go on hold - or just a single vote from a moderator can do it.
Votes don't expire, but they can be retracted by the casting user - at the cost of not being able to then vote again on that question.
Once a question is on hold it can be voted to be reopened - again I think the same thresholds and quantities apply (and a moderator can reopen it instantly).
After a while, an on-hold question will be closed automatically (again, not sure of the time involved here), and a closed question with no answers will be deleted (after another timeout period I believe).
"protected questions" are another matter entirely. A question can be "protected" so no more answers can be posted unless you have enough rep. That prevents any "me too" answers. It's not something that is used often, especially around here.
See Help Center > Privileges > Cast close and reopen votes
It's the result of "close" votes being cast.
It takes 5 close votes to reach the closing threshold ...
Close votes age away harmlessly if the threshold is not reach after a number of days. ...
Once a question has been closed, it will no longer accept new answers and the words [on hold] will be appended to the title. After 5 days, if the question remains closed, the word [closed] will instead be appended to the title.
So "on hold" really means "this is going to be marked closed soon, unless you can convince people to re-open it". However it is still actually closed.
... any user with the "protect questions" privilege can put question on hold ...
No, that's different. I've protected a couple of questions which were intended as reference questions, when people were starting to "answer" with their own problems.
Right now, users with 500+ rep will have the "cast close and reopen votes" privilege, which is what is needed for voting to close.
Moderators can also lock questions and answers, like this rather amusing one in order to prevent people altering it.
