[rick-slo]

It is the first millisecond or two of string vibration (perhaps even before you consciously hear anything) that sets up the harmonic content the string will produce as it continues to sound, and thus it is critical to tone. That first millisecond or two is determined by where and how the the finger tip leaves the string. With fingernails, since most tend to curve towards the palm, a slight backward bend of the end joint of the finger tends to smooth out the sound, and that backward bend tends to happen more with rest strokes than free strokes. Of course you can also try and do other things, for example ramping the shape of the nails, play with short nails, play with artificial nails, pick at an angle instead of perpendicular to the string.