That's a method of catching another strikers jab or pushing down his front guard, it disrupts the other fighters rhythm and stops the slow jab from getting off. The solution is to double or triple jab or hook off the jab. Or as Chung Lee did side kick under or over the lead hand.
Boxers do this all the time against the slow power jab, but usually boxer rotate their body more and use better footwork to setup their strikes. This method can be used to setup the righthand over or straight down the pipe, it can also as mentioned be used to pull in or duck under to shoot for a takedown. Wrestlers use a probing hand to the forehead to stop takedowns or forward advancement much like a football player stiff arm, so they are not tapping each other. A unconditioned person can be stunned by these taps to the thigh, calf and push away or off balance by the front probe.