They either watch you to know when to signal, in the cases where it actually “works” (which, as fucked up as it is, reading your body language is “working” for some definition of working), or they’re just using it as an excuse to search/harass you, in which case they just signal when they want to.
Fun fact, if you’re white, you can easily smuggle things by just not watching the search.
The cop is watching you to know when to signal the dog to alert.
They either watch you to know when to signal, in the cases where it actually “works” (which, as fucked up as it is, reading your body language is “working” for some definition of working), or they’re just using it as an excuse to search/harass you, in which case they just signal when they want to.