The "Soft Start" function is created exclusively for big motors and is only limited recommended for usage with a (racing-) quadcopter. Soft Start, as the name suggests, let the motors run more gently, so this can be too slow for jagged maneuvers. 

"Soft Brake" is the matching counterpart to "Soft Start"

 
"Slow Start" is a function for starting the motors slowly, which makes the whole thing look "smoother" and allows the motor directions of rotation to be checked before the flight.