If you need to trigger an event after a delay, or on a regular interval, FlxTimer can help you achieve this.

timer.start(interval, event, loops);

Demonstration

package;

import flixel.FlxG;
import flixel.FlxSprite;
import flixel.FlxState;
import flixel.util.FlxTimer;

class PlayState extends FlxState
{
	private var sprite:FlxSprite;
	
	override function create()
	{
		super.create();
		
		add(sprite = new FlxSprite('assets/sprite.png'));
		sprite.x = (FlxG.width / 2) - (sprite.width / 2);
		sprite.y = FlxG.height - sprite.height;
		sprite.acceleration.y = 200;
		
		var timer:FlxTimer = new FlxTimer();
		timer.start(5, bounce, 0);
	}
	
	override function update(elapsed:Float):Void
	{
		super.update(elapsed);
		
		sprite.y = Math.min(sprite.y, FlxG.height - sprite.height);
	}
	
	private function bounce(_):Void
	{
		sprite.velocity.y = -200;
	}
}