This site says you enter the following into the terminal and restart Safari:

defaults write com.apple.Safari DebugSnapshotsUpdatePolicy -int 2

Revert using:

defaults delete com.apple.Safari DebugSnapshotsUpdatePolicy

The site says it works with Safari 4. Let’s see if it works with Safari 5.

See also “Secrets” at BlackTree.

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via blog.vucica.net