After Palmer makes the facial cast of Rabb, Rabb's face is covered in powder. The powder appears and disappears between shots, all the while Rabb is still tied to the chair.
When impostor Rabb is questioning the colonel he reaches into his jacket for the pen. There is a cut of Mac and then back to "Rabb" reaching into his jacket again.
Palmer sedates Rabb with some type of drug, which he injects subcutaneously (under the skin), Rabb appears to be instantly unconscious even before the needle is taken out. Sedatives injected under the skin typically take several minutes to reach full effect, a person would begin to feel the effects within 20-30 seconds depending on the exact drug, but would not pass out for a least a few minutes. Even when given intravenously powerful anesthesia drugs like propofol take a good 30 to 45 seconds to render a person fully unconscious.