From the manufacturer's website:
So many trails, so little time for itchy problems, right? Tecnu® Original Outdoor Skin cleanser is a poison ivy soap designed to wash away the rash-causing oil (urushiol) from your skin (and other items) after exposure to poison ivy, poison oak, & poison sumac plants.
If you know, or think you have been, exposed to poison ivy plants (or poison oak and sumac) apply Tecnu Original to dry skin, rub vigorously for two minutes, then rinse with cool or tepid water. Although it is a soap, it can also be used without water. If you do not have access to water, you can remove Tecnu Original by wiping it off with a cloth.
The magic rule with Tecnu and poison ivy oil is, the sooner you wash the better! Wash within 8 hours for your best chance to avoid a poison ivy rash. After 8 hours, the likelihood of developing a rash drastically increases. Remember, even if you do break out, wash with Tecnu to remove any excess oil that may be on the skin; this will keep the oil from spreading to other areas.
*These statements have not been evaluated by the Food and Drug Administration. These products are not intended to diagnose, treat, cure or prevent any disease.