This slew brings the spacecraft back to the default cruise orientation and adds an additional slew to prepare for the swingby, during which:

• all Rosetta instruments are switched off; only the lander instruments will operate
• all Rosetta instruments are rotated into shadow before the spacecraft actually swings behind Mars (they will therefore not 'see' any difference and there is no risk of an unwanted automated reaction
• Rosetta instruments are pointing in the correct direction to observe Phobos when Rosetta exits the shadow of Mars, without additional slews