A short demonstration of some techniques that can be used when fighting with a spear and a scutum (Roman legionary shield). The spears used in this video are sharp. A difference between the effectiveness of the spear used in underarm and overarm grips are clear.
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Technique one. (0:050:20) Overarm spear tip threatens the head of a fighter, thus inspiring opponents shield lift. The spear is reaimed to trust the shield downwards, thus "locking" it on the spot. On the next tempo fighter who has committed the trust bashes his shield to the "locked" shield, freeing his spear with this same move. Fighter who does this is now free to use one's spear, while still protected by shield and locking opponents shield (and sometimes body) in place.

Technique two. (0:200:30) When trusted towards face the fighter defends by blocking with the spear shaft that was readily in the overarm guard. Thus he moves the opponent's spear aside while still aligning one's own spear against the opponent.

Technique three. (0:330:43) Punching opponents shield/body with shield to gain control. Opponent tries to parry with a spear thrust, but is in the underarm guard from where this parry is very unlikely to succeed.

Technique four. (0:440:54) Parrying a shield punch from overarm guard, pushing the opponent's shield downwards thus stopping the forward movement. Next tempo has the opportunity to continue similarly than in technique one.