El Chinito and La Piedra are anesthetics that are rubbed on the penis to lower the sensitivity of the penis and allow a man to maintain a longer time of erection without ejaculating. They use similar products in hospitals to numb patients before injecting IV or during dental procedures.
Contrary to the effects of Viagra, which increases blood flow to the penis and helps get it up for men who can't do so on their own, La Piedra and el Chinito are geared for men who can get it up, but are A) ultra sensitive or B) want to really go at it for hours.
El Chinito comes in a small package about the size of a marble. You take a small bit and wet it creating a paste. You rub the substance on the tip of the penis about a half an hour before penetration. Usually the man would rub the stuff on and then commence cunnilingus on his partner until the effects kick in. Mind you, the amount should be very small as the effects are strong, since it basically puts your penis to sleep.
The effects wear off eventually, though a man should have a condom on as the effects on a female are unknown. In some cases it could cause irritation.
The small rocks are common and easy to find and are relatively cheap. You can go to small herbal and health food stores as they will usually have them.
If you have any questions just ask.
Hope this helps,
Mr. Lu