The clutch pack is a pancake stack of about a dozen disks, alternating friction disks against plain metal disks. The friction disks are attached to the input shaft by splines on the inside. The plain metal disks are attached to the output shaft by splines on the outside. When the clutch slips, the disks move at different speeds. If you pound some nails (dowels) into this stack of pancakes, they all move together at the same speed. As long as at least one disk from each (input and output) is pinned with a dowel, any disks without a dowel will still move together since it's splined to its buddies. The disks on the ends have no dowel holes simply to keep the dowels from falling out.
See the green #3 in the picture? I just drilled a hole right where that row of green dots is.
