The most accurate description of what a rise does is that it falls more slowly than a regular pitch. More slowly than the brain expects it to, thus the "jump" illusion.
It is physically impossible for a human to throw a softball - over or under handed - with enough speed and spin to achieve upward movement. The ball is too heavy and the seams too small.
There is a bit more to it. Yes it stays up and does not rise. However, do not discount the rapid deceleration which attributes to the jump of the riseball or late break of other pitches. The greater the spin rate the more it decelerates. If you clock pitchers with a Stalker II or other higher end radar gun you can measure the loss of velocity from release to the catchers glove.
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