some doubts / questions

[nrage]

to generate more spin is how fast, how low and far you can deliver you cue in straight line in my opinion

Exactly. For some people using a soft grip lets them do these 3 things better. For some people a hard grip means more tension in the arms and less speed. For some people a hard grip means lifting or dropping the cue tip and striking higher, or miss-cueing. For some people a hard grip means less follow thru, stopping earlier. And some people have no trouble with any of these things with a hard grip, so see no difference with soft and hard. Personally a soft grip really helped my screw shot.