I know these are in use on many forums (especially forums regarding films, TV shows, etc.), what they do is simply hide the text within them and replace it with a "show spoiler" button, which when clicked, shows the text.

On a forum like this, there could be two uses for this feature: firstly to hide plot spoilers (their normal use), but also to hide translations. For example, if someone wants help with their Russian, people could provide a reply in Russian followed by a reply in English in spoiler tags.

Example:
Original post:
What case should I use when replying to the question «откуда»?
Reply:
Используй родительный падеж.
[spoiler]Use the genitive case.[/spoiler]