Soft: The initial oscillator phase starts at a zero crossing for each played note. This mimics the sonic character (and precision) typical of digital synthesizers.
Hard: The initial oscillator phase starts at the highest level in the waveform cycle for each played note. The extra punch that this setting can provide is audible only if the ENV3 Attack Time parameter is set to a low value—a very fast attack, in other words. This setting is highly recommended for electronic percussion and hard basses.
Note: Osc Start soft and hard result in a constant output level of the initial oscillator phase every time the sound is played back. This may be of importance when you use the Bounce function of Logic Pro at close to maximum recording levels.
Download the guides:
Logic Pro User Guide: PDF
Logic Pro Effects: PDF