The imprinted stamp on K's head enables Bengali to track him anywhere. Aside from that "simple" answer, your other questions require more detail...
I've not seen the film, but after some basic digging I'll try and answer some of our questions. It does appear to be an extremely bizarre movie that was critically panned for a number of reasons, including a disjointed plot which didn't always make sense. To try and answer some of the key questions you have:
- Baba Bengali is able to forcibly make people stop smoking. Part of his genius is that he always knows when people have smoked, regardless of where they are in the world.
At least that's the way the film makes it out to be.
Effectively, at its most basic level, the film is about smoking (obviously!). Our protagonist, K, is a chainsmoking narcissist who risks losing his wife if he doesn't stop smoking. He is recommended a visit to the Prayogshala, the rehabilitatoin center that never fails, by an old friend who originally introduced him to smoking.
K ignores all of this however and so his wife leaves him.
He is now forced to accept the treatment (against his will), as he has been left alone and wants her back. When he reaches the Prayogshala, it is evident that it is a kind of hell, where many mystic happenings are taking place.
Baba Bengali referred to the Prayogshala as, "Patal Lok." In Hinduism, that apparently means the world underneath us.
K is forced to sign a contract and has an antismoke stamp placed on his forehead. It appears to be through this stamp that his actions and whereabouts are always known to Baba Bengali.
There are four levels of punishment for if/when he smokes:
- The first offense is almost killing a loved one by keeping him/her in
a chamber full of cigarette smoke the person has smoked in his entire
life for a duration of 5 minutes.
- The second offense is losing a
- The third offense is the death of a loved one.
- The last
offense is taking the soul of the person from his body
As the film progresses, K fails twice and receives the first two punishments. He is then offered a lit cigar but refuses. Baba Bengali makes a mistake here however and believes he has accepted it. He takes K's wife, the third punishment. When K tries to angrily explain Baba Bengali's mistake, he is apologetic but relentless. K is broken at this point. Infuriated, bewildered, he calls the police. They don't believe him and force him to light another cigarette. Shortly afterwards, he discovers his brother has taken his own life (the "real" third punishment, since his wife was an accident).
At this point he snaps awake in an army base. He escapes from it and ends up stumbling upon a room where he appears to "accept" his treatment. Smoke fills the room, he wakes up in the real world, goes to the bathroom and realises he is missing two fingers.
Movie Explanation (or attempt thereof!):
Siberia - At the start of the film, K dreams about Siberia. This is a complete wilderness, symbolising his desire to escape his environment and be free - but even there, he cannot escape the smoking. It seems very likely that at the end of the film, when he wakes in the Russian army base, he is again in Siberia - but this time, the wildnerness has crumbled apart. There are others there with him and he realises this is the end. He must return to the real world and embrace change (i.e. give up smoking).
Sunglasses/Bathtub - K's sunglasses appear to symbolise his dark and gloomy life, whilst the bathtub is his place of pleasure and relaxation. It is the one place his mind is calm. Therefore, when he goes there at the end of the film and realises he is missing his fingers, he is realising this change calmly - in his environment of serenity. In other words, he is accepting it.
1 Rupee - The significance of the 1 rupee in the film, the small amount K couldn't afford to pay, symbolises the tiny things that all must be done to effect change. Without that last rupee, his treatment is incomplete. Similar to an alcoholic not attending AA meetings, for example.
Two fingers - Note that the fingers K loses at the end of the film are his smoking fingers. This symbolises his acceptance of the need to quit smoking. He is prepared to forsake his fingers (whether temporarily or permanently) to embrace change and improve, both for himself and his wife.
Hearing aids - Note at Prayogshala, all the patients have hearing aids. This symbolises their inability to listen to repeated requests to seek help and their need to be helped to achieve any level of treatment.
The movie does appear to be extremely convoluted. Some have hailed it as a Lynchian masterpiece, but the consensus appears to be that the scenes do not all work in particularly great harmony. Was the whole film a dream? Was just the core part of the film a dream? Did he really lose his fingers at the end or was that just a symbolic scene? It's impossible to say (and the director hasn't commented).
Ultimately what is true is what you want to believe is true - something that always goes along with these psychedelic movies.