Armor spikes' page says:
Armor spikes deal extra piercing damage (see “spiked armor” on Table: Weapons) on a successful grapple attack. The spikes count as a martial weapon.
Which seems to imply that it functions similarly to Constrict; extra damage on a successful grapple check.
But Grapple says:
You can inflict damage to your target equal to your unarmed strike, a natural attack, or an attack made with armor spikes or a light or one-handed weapon.
Which seems to imply that they are just another option to use in a "Damage" action from a successful check.
Which is the correct usage, and if it's the former, would it still apply the grappler's strength mod to damage?