The mode recovery mode and download mode are useful to restore your Samsung Galaxy device to factory conditions or to manually update the firmware with Odin.
Here's how to enter download mode mode and recovery mode on Samsung Galaxy S2, See, e Samsung Galaxy Tab 2:

Mobile_odin_logoSamsung Galaxy S2:
Download Mode: Simultaneously press Power + DOWN key + Home (middle button)
Recovery Mode: Simultaneously press Power + TASTO WITH + Home (middle button)

Samsung Galaxy Se:
Download Mode: Simultaneously press Power + DOWN key + Home (middle button)
Recovery Mode: Simultaneously press Power + TASTO WITH + Home (middle button)

Samsung Galaxy Tab 2 10.1:
Download Mode: Simultaneously press Power + Volume SU (right)
Recovery Mode: Simultaneously press Power + Volume Down (left)