The whole idea behind a barefoot trim is to keep your horse landing heel first (so the hoof lands flat and absorbs concussion) while keeping the coffin bone ground parallel upon landing.
In other words, since the coffin bone's (P3's) shape mimics the greater hoof capsule, the tip of P3 should not be pointing down into the sole or up away from the ground. Either one causes lameness, for obvious reasons.
A well-trained hoof professional can tell how P3 is oriented within the hoof capsule by measuring the collateral groove on either side of the frog-a deeper groove in the back could mean that P3 is tipped forward and is often accompanied by some wall separation in the toe-this horse could be at risk for founder if he isn't experiencing symptoms already.
Or the opposite situation could show itself, where there is a shallow collateral groove at the back of the foot, and too much sole at the front of the foot. This negative palmar angle is equally painful and can be quickly remedied with a knowledge of the internal structures through measuring that groove.
Trimming with the knowledge of the internal structures (the closest thing to an x-ray) can be very groovy for your horse indeed.