what is A2SD+ and how to use it?

I'm using the MIUI ROM on my HTC Desire and it has an option called A2SD+ which is supposed to be an SD partition that the device treats as internal memory.

I don't know if it comes from the ROM or from Android (2.3.4).

How can I use it? Do I need to partition my SD somehow with an outside tool like I saw on some posts here? If yes, how do I then tell my device that the 2nd partition is for A2SD+?