Moving left to right is a bit more tricky to get into the tunnel. Land aiming down approximately 3 pixels inside the leftmost tile. If Samus is too far left she will fall out, too far right and a thorn pushes her out, not aiming down puts her stuck in crouch. Then spinjump right to take a thorn hit and get wedged in the ceiling. Then start the crawl while taking constant thorn damage. Perform one jump per thorn hit. Note that if Samus is hit by the thorn before landing, aiming down is not necessary. Note that a moonfall may be helpful to get into the tunnel. Without moonwalk, a downback can be used instead: fall off with low speed and start a downback right away. (This doesn't always work.)