Mongo server operations emulation - so you don't have to reload your document.

Most of mongo field functions like inc, push, pull etc. can be emulated, so you don’t have to reload the document to see the changed values. Actual document processing is of course still done on the server.

You can enable it by default by setting $_emulate to true, or by using set_emulation(true).
Additionaly, every emulated function accepts $emulate parameter.