Unlike normal redirects, a soft redirect does not use special code or functionality of the wiki software. All they consist of is a notification to users, showing the destination to another site to obtain the information they were seeking.
The technique is particularly likely to be used when redirecting users across Wikimedia sister projects—for example Wikipedia:Gather is a soft redirect to mw:Gather. Normal redirects would be undesirable in these circumstances, and hard interwiki redirects are disabled. (Reasons: they could not be easily edited without hand-crafting the correct URL, since clicking on a link to the redirect page would take you straight to the redirect's target and there would be no "Redirected from foo" message to click, so it would be difficult to return to the redirect page itself; there would also be infinite-loop security considerations.)
Another situation where soft redirects are used is when the intended target is a special page, and the system automatically "softens" attempted hard redirects to special pages. For example, Wikipedia:List of tags redirects to Special:Tags.
Soft redirects to non-English language editions of Wikipedia should be avoided because they will generally be unhelpful to English-language readers.
Soft redirects are intended mostly for external use, where hard redirects will not function. For internal use in general, hard redirects should be used instead.