this technic is easy to do and yet is hard to figure out for some. This works best in an editor like Sound forge, Wavelab, or Audacity. First take the sample you want to use then load it into your editor. Reverse it. After this load it into a new project that is at the same tempo as the main one. After you do this add a delay or vocal. The longer the better usually because this can work on a build to a main beat. After you add the desired delay or reverb make a seperate channel that is empty and copy and paste it to the point where your reverb or delay ends. Next after it is rendered reverse it again. Now you have a backwords delay or reverb. You have to think of it backwords by when the sample would kick in. If you Arrange it just right it will site in your track well