I do not know the science behind it. But from my playing experience in BD, I sometimes able to swing the new ball but neither knew why it was swinging nor could repeat the swing in the next ball.
I did ask to some elder brothers who were playing in premiere that time. They showed me the difference between grip and wrist position for in-swing/out-swing. I tried it, sometimes happened but could not make it happen willingly. Then I talked to them about this, they said even some top bowlers not sure about the ball swing or not. Even it happens that bowlers trying to ball an out-swing but it actually ended up been in-swing. So, my experience was always "jodi hoia jai", but happened a little. But I always found it more probable to swing in grass than in the pitch.
My conclusion is, weather wise you cannot be sure about the ball will swing or not. It may or may not swing in overcast/cloudy condition. But if there is grass, you should be sure to get something out.
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