DRV8908-Q1: Negatie-current OLD and synchronous rectification mode

Hi team

I am confused on the negative-current OLD and synchronous rectification mode, could you help answer the following questions? 

1. What is purpose of synchronous rectification mode? And when to enable this mode? 
2. What is the status the motor is at for such a PWM waveform? OUT1_HS is always high but OUT2_LS/OUT2_HS is complementary?