A trick is euphemism for a magic spell or work, but usually an action or ritual performed with an item that is thrown, laid down, buried, hidden, or otherwise having a connection to the ground, in the practice of hoodoo/rootwork.
Examples of tricks include; throwing powders, burying bottles, packets, doll-babies, or mojo bags, or hiding a mojo bag or packet in the home or car of a person. (Note that objects hidden in the home or car don't really involve the ground, but they are often hidden "low", save for those hidden above a doorway or fireplace mantle.)
When a trick is "laid" it is customary for the rootworker to walk away and not look back. The belief that one should "not look back" is quite old in the practice of magic. The ancient Greeks held to similar beliefs, especially with regards to leaving behind offerings to the goddess Hecate.
The belief that one should not look back after laying a trick may ultimately stem from an ancient African fear of offending a spirit or deity or it may simply be not wanting to second-guess one's work.