Weight and grip of sarissa and shield in macedonian phalanx

[Paralus]

It's fairly certain that the details of the battle relayed by Plutarch were derived from Scipio Nasica's firsthand account, and so they can be taken as trustworthy.

I'd agree that whilst Plutarch is working from both sources here - Nasica and Polibius - the battle detail is almost certainly Nasica's as it is extremely doubtful that Polybius was anywhere near this battle.

What next he sees is more likely the as yet to engage phalanx brigades lowering sariae and advancing to the attack. Hence the appearance of the slung shield "rotating around and down" with the movement down and forward of the arms?

Nick Sekunda, whose analysis of the battle sounds the most convincing to me, asserts that the troops in the second line described as swinging their peltai around were peltasts following up the first line after their engagement with the Romans.

Now that opens up the question of just what does one describe as a "peltast". In any case, it matters not: Plutarch clearly describes these forces as lowering their sarisae. They are phalangites no matter whether we term them that or "peltasts". To my reading there has been an "incremental" engagement. One side of the field is likely engaged before the other (or units). The battle detail is lost in Plutarch's very compressed narrative - remember, for this writer, Paraetecene was not worth recording - who is more interested in telling us about the man Paullus and his five sons. It is most likely that almost separate engagements were going on the field - particularly with a river between the combatants. The description of the Macedonian phalangites lowering sarisae from the near vertical to horizontal - and the "targets" or shields following round and forward - also speaks to an eye-witness.