So the calcium latches on to the troponin and causes it to pull the other bodyguard protein — the tropomyosin — away from the sites on the actin strands that the myosin really wants to get its paws on.
Immediately after a muscle fiber is stimulated by a nerve — when calcium ions are flooding into the sarcomeres to pull away those two protein bodyguards of tropomyosin and troponin from the actin — that's called the latent period.