Hide Overshares is a feature designed to balance the upload of chunks within a file. It is especially useful for releasers who wish to spread the file in as few uploads as possible, but it could be useful for any responsible uploader.
Abbreviation : hideOS
It works by hiding chunks that were uploaded significantly more than other chunks from downloaders. This causes downloaders to request other, less shared, chunks. If all chunks were uploaded equally, all chunks are shared. The exact number of uploads that qualify as 'significantly more' is selected by the uploader.
For example, a releaser may choose to set overshares to 2. Then, if one of the chunks is fully uploaded twice, it is hidden from the people downloading the file. Once the remaining chunks are fully uploaded at least once, the chunk is shown again. If that chunk is uploaded once more, then it has once again been uploaded 2 more times than the remaining chunks, and it is once again hidden.
If overshares is set to 1, each chunk will only be uploaded once until the file has been completely uploaded, and then the process restarts.
Note that the measure of whether a chunk is overshared is done by comparing it to the least uploaded chunk. This means, on one hand, that an overshared chunk does not remain permanently hidden, but becomes visible again once the rest of the file has been uploaded. On the other hand, it means that if a file is only partially uploaded, it is not enough to make an overshared chunk visible, since the file has to be fully uploaded.
Newer implementations of the feature ignore chunks that cannot be uploaded when looking for the least shared chunk. These include unavailable chunks from an incomplete file, and chunks the downloading clients already have. This is to ensure at least one chunk that can actually be uploaded would always be visible to other clients.
It simply preserves balance within the part of the file that can be uploaded, while allowing that part to become unbalanced compared to the part that cannot be uploaded.
The actual measurement of how many times a chunk has been uploaded is based on the spread bars information, so this feature cannot exist without spread bars.
(cyan is hidden, white is not hidden and black is what the other client has).
This feature was originally created by SlugFiller.
 Selective Chunk Sharing
This strengthened version of Hide Overshares reveals only a single chunk to the user. When more than one chunk is sharable, a random chunk is chosen to be shown to the user.
Each user is shown a different chunk, if possible, and when a chunk must be shown twice, the least-shared chunks get first priority.
Since all users are forced to download a unique part of the file, the end result is that all parts of the file become shared in a balanced manner. This method is more pre-emptive when compared to standard Hide Overshares. Hide Overshares only ensures balance after the file is uploaded partially, while Selective Chunk Sharing makes sure that the file is downloaded in a balanced manner from the start.
Like Hide Overshares, it ensures the revealed chunk can be uploaded to the downloading client.
 See Also
You should also look at Share Only the Need as well if you are planning on using HideOS.