During the closure interval for a (non-nasal) stop consonant, the vocal tract is completely closed, and no sound escapes through the mouth.
However, at the moment of release of the stop constriction the resonances of the vocal tract change rapidly. These changes are traditionally called formant transitions.
The first formant exhibits a rising transition after the release of a stop closure.
The direction of the second and third formant transitions depend on the particular constrictor producing the stop (lips, tongue tip, tongue body), and also on the overlapping vowel.
Early research on the perception of stop consonants
The device called the pattern playback allowed researchers to trace through a spectrogram and convert the resulting drawings to sound.
Using this device, it was discovered that the direction and extent of second and third formant transitions were sufficient to determine the perception of "place of articulation" (constrictor type).