They are talking about non rubbery surfaces > When encountering a smooth, stiff, and impermeable surface, such as polished metal or glass, the actual contact a

They are talking about non rubbery surfaces > When encountering a smooth, stiff, and impermeable surface, such as polished metal or glass, the actual contact area is initially small as is the friction. Because the keratin softens when it is hydrated by the moisture secreted from the sweat pores, it requires many seconds for the contact area to increase to the value reached almost instantaneously with a soft material, such as a rubber.