If you were standing behind the Union troops in the very center of their line looking west toward the Confederate battle line, you would be in the bottom of a “V”. Literally, there would be a very steep hill to the left and right of your position.
So to keep moving forward and staying in touch with the regiment on both sides was a very hard thing to do. Union troops on the left side of the “V” had to climb up a hill, go down another hill and up the side of a steep ravine, while under musketry and artillery fire from the Confederate. The troops probably had to strap their muskets to their backs to free their hands for climbing out of the ravine. That they accomplished this feat was absolutely amazing.
On the right side of the “V”, the Union forces had an uphill advance, but without quite the up and down movement of their brothers to the south. Steady pushing back the thinly defended Confederate battle line, the Union advance continued unabated. On the left side of the “V” the Union force pushed the Confederates back to the southern edge of the town of Sharpsburg.
If the Union troops could turn the Confederate flank, they could cut off the only ford available to General Lee to cross the Potomac and get away from the Union Army of the Potomac.