Plumbata

[Robert Vermaat]

On the subjects of throwing method and tactical use I'd like to offer up some experiential and conjectural thoughts.
Underhand vs. overhand:  [..] Having spent much of my life on a baseball field as a player and coach, I have never seen any player resort to the underhand throw to achieve either maximum thrust or accuracy.  [..]
The underhand style does have a couple of advantages, the biggest of which is that we humans are built in such a way that underhand throws cause less stress/fewer injuries to the thrower.  The other is that that a 'lob' throw (high arcing, short 'ballistic' trajectory) is much easier to achieve underhanded.

Hi Bruce, I heard it snowed in the mountains?

Two things about throwing plumbatae:
1 - the dart is not a perfectly balanced object and it does not behave like .e.g an arrow or a well-made spear (or a ball). Athough it can be aimed over short distances (where throwing overhand is recommended), this is generally recognised as a mass-thrown weapon the aim is not an individual target.
2 - the preferred impact of a plumbata is from a high angle. As you observed, the arching throw is much easier to achieve underhanded, which is probably why most of the modern testing almost always achieves larger distances with underhand throws.

I do maintain that the primary style would be overhand, especially in battle formation, for the following reasons:
First of all, a formation using the underhand style would, by necessity, have to be very loose because that method requires more space. Measuring the travel distance of the dart's point for each style proves that. [..]
Secondly, in light of the above, the training doctrine/regimen for underhanded throwing would necessitate new formations and commands. Throwing darts overhand, conversely, is essentially the same as throwing pilae.... same motions = same training.

You are basically correct, and opinions vary. However it can be even more simple. Plumbatae were carried by everyone but not necessaily thrown by everyone.

Not all ranks need to threw at once, so they did not need to open up in order to avoid hitting the man in front - although commands are available to do just that.
To achieve maximus distance (underhand throwing) it's probable that just the front rank threw before the opening of the battle. When the formations were in contact, it's more probable that plumbatae were thrown behind the front ranks, according to Roman doctrine of keeping up a constant rate of fire into the enemy (also slingers and archers did that).
It was of course possible to deliver a massed throw (overhand) from the front ranks when attacked by cavalry or in similar situations.