A shot put arm swing is one where a player uses more of an upward pushing motion (like a shot putter) to contact the ball than a throwing one. This is most often seen in spiking, but it appears in serving as well sometimes. The result is that the player strikes the ball from underneath with an upward trajectory.
So how do we correct this?
Look at their throwing mechanics
In many instances player shot put because they never developed a proper throwing motion. This is one reason I always advise teaching them how to throw, especially when working with younger athletes. There is, of course, more that goes into a good volleyball arm swing. Just looking at the throwing motion, though, can take you a long way to addressing the shot put action.
Have them hit standing down balls
Since we’re talking about hitting here, looking at throwing only takes you so far. The player has to work on the throwing motion in conjunction with a moving target to strike. Self-tossed balls is a decent starting point, but you definitely want to move on to stuff like over-the-net pepper where live balls are involved, as I mentioned in this post.
Work on their approach and timing
Most of the time when we see players use a shot put arm swing it paired up with an approach to suit. Basically, the player runs under the ball, jumps straight up, and oftentimes uses that jump as the launch of their upward ball push. Because they strike the ball at a lower point (down near the shoulder rather than above the head), the timing for this type of hitting is slightly later than for a proper arm swing. So you’re likely going to have to adjust their approach mechanics some, while also reworking their timing ot the ball.
Because players who naturally shot put don’t really have any sense of what a proper arm swing should feel like, you need to help them create the link. Using video can help this a lot. It allows the player to contrast correct vs. incorrect motions. You don’t need anything complicated. A phone or tablet will do. There are apps out there that will let you do side-by-sides.