Is about a 45 degree angle of elevation for the cue correct?
For most practical shots, it will be less than 45 degrees. It depends on how much of the ball you need to clear and how soon.
Should I hit the cue ball right in the center or a little above center?
Below center is better, but not so low you miscue, which is a foul on jump shots, at least at nine ball. If you hit above center, the cue stick tends to trap the cue ball on the cloth.
Rule 3.24 says it is illegal to "dig under" the ball to get it to jump. Hitting the cue ball below center is not "digging under". By "digging under" I assume they mean a miscue. Miscues are illegal by rule 3.25.
Should I stroke through the cue ball, or does that interfere with the cue ball jumping?
You need to use a somewhat shorter stroke to avoid hitting the cloth. If you are already slowing the cue down at the instant of contact, it will act as if it were lighter, which is better.
Does it have to be hit extremely hard?
It depends on the distance from the cue ball to the obstruction, the weight of the cue stick, and how much of the obstruction you need to clear. The cue ball's path while in the air is a parabola, and you can calculate how fast the ball must be going to just clear the obstruction at the peak of the trajectory.
The most important factor is the kind of cloth on the table. If it is very high quality, thin cloth, jumping will be very difficult. If it is thicker or maybe rubber-backed, jumping will be easy.
Start with an easy drill: Freeze three balls together in a line parallel to and about a foot from a rail. Remove the middle one. Place the cue ball an inch from the rail, and shoot it through the hole. Twenty degrees elevation should be plenty for this shot. Do the same, but place an object ball to be pocketed after the jump. Move the two obstructing balls closer to each other and/or farther from the cue ball.
At snooker, it is a foul for the cue ball to jump over a ball, whether intended or not (unless the cue ball has already struck an object ball)
