This is a video taken of a string being plucked. Before the string is released the force provided by the fingers is balanced but the tension in the string. The string begins to accelerate once it is released, but only when the string is unstretched (flat) is there an unbalanced force. As a result the string deforms first to a trapezoid until it is flat. The motion of the string at each point can be described by the sum of oscillating standing waves with nodes at each end of the string. You can clearly see a number of these waves during the first few cycles. Eventually only the longest wavelength (fundamental) mode remains.