To process a negative instruction (e.g., don’t fall down), the child must access some representation of “falling down.” That internal representation, especially if it is kinesthetic, will usually result in the behavior you are trying to prevent. A positive instruction (e.g., be careful, move slowly) will access a representation that will help the child cope with the situation. (Bandler, Richard, and John Grinder. Frogs into Princes. p 64-65. UT: Real People Press. 1979.)
Refer to Affirmation and the Brain for additional information.