I'm a nurse and working in emergency and critical care most of my career. I can easily observe the pulsations of your jugular vein's when people are in the later stages of heart failure, I was surprised to observe the same on many players at times of stress in card games. It's not 100% reliable as some people have some level tachycardia when both bluffing as well as playing a strong hand. Then I thought hmm if I can notice this then so can others so I tried practicing "faking" it. To me it is barely passable but I have heard others specifically commenting on my heart rate, while I was faking.....then after laughing how even that was a crazy bluff....
Pupil Dilation, eye movements, diaphoresis (sweating), body movements, yawning, etc are all phsiological responses, from various stimuli. Someone who is deeper into the cups will have a different appearance giving away their level of consumption, this can be exploited if one wished. Eye movements have a statistical average based on thought processing. Certain eye movements paired with behaviour can give a suggestion as to the thought processing involved and whether players are speaking or acting truthfully, deceptively etc... Again because 1 player yawns after the flop when they have a strong hand doesn't mean that another player will or the same player will yawn again next time. Its about gambling with as much angle information as you can possibly gain or at least that perceived edge whether real or not.
It sounds great, but know that I see the same health information everywhere I go.....Standing in line at the grocery store I can see a person who has chronic COPD based on the sound of their breathing, resp effort or shape of their fingernails.....level of alcohol consumption based on smell, behaviour, appearance and even sometimes speech....Rheumatoid arthritis.....Cancer.....Drug Use etc..... Sometimes I wish I could and actively try to "turn it off". Pro's and Con's