How was Absalom trapped in the battle in the forest of Ephraim?

Answer: His head caught in an oak tree

As his mule went under a large oak, Absalom's head was caught fast in the branches, and he was left hanging in midair.

“Now Absalom was riding on his mule when he met the servants of David, and as the mule went under the thick branches of a large oak, Absalom’s head was caught fast in the tree. The mule under him kept going, so that he was suspended in midair.”— 2 Samuel 18:9 (BSB)