The phrase "winds down" refers to the gradual decrease or conclusion of an activity or event. Antonyms of "winds down" would be words that denote increase, acceleration or prolongation of an event or activity. Some suitable antonyms could be "ramps up," "accelerates," "extends," "prolongs" or "expands." For example, the company's advertising campaign ramps up as the launch date approaches. Alternatively, the party accelerates as the night wears on. Rather than winding down, the excitement extends into the wee hours of the morning. Therefore, the opposite of winding down would be to ramp up or prolong an activity.