How did Henry's regiment begin their advance?

They began their advance slowly. Grouped together they moved (strained)to move in their formation. As the men moved forward they were leaning..... getting ready for the charge. Use clues from the text to envision this group of men standing at attention and moving forward and suddenly looking like they've toppled (see the crumbling wall?). When the wall goes down, they begin to run.... the charge is on.

"The youth stared at the land in front of him. Its foliages now seemed to veil powers and horrors. He was unaware of the machinery of orders that started the charge, although from the corners of his eyes he saw an officer, who looked like a boy a-horseback, come galloping, waving his hat. Suddenly he felt a straining and heaving among the men. The line fell slowly forward like a toppling wall, and, with a convulsive gasp that was intended for a cheer, the regiment began its journey. The youth was pushed and jostled for a moment before he understood the movement at all, but directly he lunged ahead and began to run."