Freehub failure - what happens?

Apologies if there is already a thread on this (I'm sure there is, but the site search isn't particularly helpful in finding it).

I have a 2013 Mavic Ksyrium Elite wheelset. The freehub normally emits a loud clicking, but recently has added an intermittent Chris King esque angry buzz into the mix - usually at speed when freewheeling on a descent. Initially I thought it was some brake rub or dodgy indexing, but I'm now pretty sure it's the freehub, and am taking it very easy, soft-pedalling down descents while on the brakes to avoid the dreaded buzzing and any hospital visits that may ensue.

But because I have no mechanical nous, what actually happens when a freehub fails? I'm imagining the cassette suddenly becoming permanently engaged, the bike effectively becoming a fixed wheel, and me being thrown over the bars. But is that what happens, or does a failed freehub fail 'safe' so you just end up with no engagement or drive?

If you're new please join in and if you have questions pop them below and the forum regulars will answer as best we can.

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