Ok, lets start with what "pin" means per the rules for grappling. From the SRD:
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Pin Your Opponent: You can hold your opponent immobile for 1 round by winning an opposed grapple check (made in place of an attack). Once you have an opponent pinned, you have a few options available to you (see below).
If You’re Pinning an Opponent
You can attempt to damage your opponent with an opposed grapple check, you can attempt to use your opponent’s weapon against him, or you can attempt to move the grapple (all described above). At your option, you can prevent a pinned opponent from speaking.
Damage Your Opponent: While grappling, you can deal damage to your opponent equivalent to an unarmed strike. Make an opposed grapple check in place of an attack. If you win, you deal nonlethal damage as normal for your unarmed strike (1d3 points for Medium attackers or 1d2 points for Small attackers, plus Strength modifiers). If you want to deal lethal damage, you take a –4 penalty on your grapple check.
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Now, on to Earth's Embrace. Earth's Embrace makes it so when you Pin the opponent, you do 1d12 damage each round that you maintain the pin.
So, if you are using Earth's Embrace a Pin Action would be thus:
Pin Your Opponent using Earth's Embrace: You can hold your opponent immobile for 1 round by winning an opposed grapple check (made in place of an attack). Once you have an opponent pinned, you do 1d12 damage each round that you maintain the pin as a free action, but you are considered immobile, giving opponents (other then the one you are pinning) a +4 bonus on attack rolls against you.
So, each round after you pin an opponent you do 1d12 damage.