To perform an inner forearm middle block, you block with the bony area on the thumb side (or inner side) of your forearm. Like the outer forearm, it spans one third of the distance from your wrist to your elbow.
Like many blocks in Taekwon-Do, you prepare for an inner forearm middle block by crossing at the wrists. The arm performing the block should be on the bottom and both backfists should face the ceiling. (Or both palms should face the ground, if you find that easier to visualize.) Your bottom arm should be just below the top of your shoulder.
The point where your arms cross should be at roughly the opposite chest line. By that I mean that if you are blocking with your left arm, the cross in your arms should be in front of the right side of your chest, just inside the shoulder. Reach your arms nice and far away from your body while still keeping a relaxed bend in the elbows.
You’ll need to twist your body about an extra quarter-turn beyond the final facing for the block. Because this block finishes half-facing the target, you should turn to about ¾-facing for the preparation.
Move your arm in a straight line from the starting point to the finishing point, rotating your fist as you go. Most of the rotation happens near the very end of the movement. Your opposite hand comes back to your belt in a fist, with the backfist facing the floor (i.e. palm facing upward).
You should finish with your backfist facing your hypothetical opponent (i.e. your palm faces you). There should be almost a 90-degree bend in the inside of your elbow. Make sure to keep your wrist straight.
The top of your fist should reach the top of your shoulder. The reason for this is that your hypothetical attacker is aiming for your solar plexus. By positioning the top of your fist at the top of your shoulder, your blocking tool is in the proper position to block the attack.
The blocking tool should finish in line with the centre of your shoulder. Also, make sure to drop your shoulder slightly at the moment of theoretical impact.
If you’re doing an inner forearm middle block against a real, live partner, you should try to finish your block in line with the centre of your partner’s chest, if your block is travelling from the outside of their body toward the centreline. This is the ideal side to block from, because it sends the attacker’s arm across their body, making it harder for them to follow up. If your block is moving from inside to outside, you’ll want to finish half a shoulder width outside of your partner’s shoulder. Finishing at this distance means that you’re putting enough power in your block to properly deflect the attack, but no so much power that you overextend the block and leave yourself exposed.
Finally, even though I’ve illustrated this block in walking stance, all the same rules apply in L-stance (or similar stances, for that matter).