Sport Mode is also known as “rate controlled stabilize” plus Altitude Hold.
It was designed to be useful for flying FPV and filming dolly shots or fly bys because you can set the vehicle at a particular angle and it will maintain that angle.
The pilot’s roll, pitch and yaw sticks control the rate of rotation of the vehicle so when the sticks are released the vehicle will remain in its current attitude.
The altitude is maintained with the altitude hold controller so the vehicle will attempt to hold its current altitude when the sticks are placed with 10% of mid-throttle. It will climb or descend at up to 2.5m/s (this speed is adjustable with the PILOT_SPEED_UP and PILOT_SPEED_DN parameters). The acceleration used to establish these speeds is set by PILOT_ACCEL_Z.