Sorry, it requires a little more explanation. As explained, the outward motion of the cluster is along a helical path. Just before the expansion starts, the motion was planar, in a plane perpendicular to the radius of the universe. As it moves outwards, the motion becomes helical. The outward component of the motion thus starts from zero, reaches the maximum when the expansion is half way, then decreases and reaches zero when the expansion comes to an end. But the planar component of the motion goes on increasing till the end of the expansion. At the end of expansion, the motion becomes planar again, and the speed is very high.

During contraction, the motion becomes helical again. The inward component goes on increasing till the contraction is halfway, then decreases and finally becomes zero. The planar component, however goes on decreasing till the contraction comes to an end. That is, the clusters never stands still, but the system as a whole expands and contracts due to the changes in the direction of motion.