As Bill Burroughs once remarked, "People often ask me if I have any words of advice for young people."
Today's Question: how risky is choking or strangling a person?
Whether engaging in erotic asphyxia or applying a "constriction-technique" in martial arts (絞技, etc), you can reduce or negate most of the risks of gross physical damage (like crushing the hyoid, trachea, or larynx) or accidental death due to gross physical injury with proper technique. Strangulation (cutting off blood) is generally regarded as "safer" than asphyxiation (cutting off air) because it is less likely to result in fractures of the structures mentioned above, and a rapid, controlled strangulation that causes unconsciousness quickly is much safer than a protracted event that causes a person to slowly lose consciousness over a period of struggling.
Regardless of technique or intentions, the two cardinal rules are to apply force with a "fail-safe" rather than "fail-secure" mechanism (i.e. constriction ceases when no longer deliberately induced), and don't apply any more force than is required to achieve the desired effect. Roughly 250 mmHg for 10 - 20 seconds is normally sufficient. This also means you don't hang yourself or anybody else, and you don't compress things until they turn purple unless your intention is to cause grave bodily injury. Accidents happen; accidents like this will have dire consequences, but you can reduce these kinds of risks to levels you may find acceptable by being responsible and respectful.
What you can't prevent are the body's systemic responses to ischemia (loss of blood flow) and ischemic hypoxia and hypercapnia (not enough oxygen and too much carbon dioxide in the blood); you will never be "responsible enough" to dictate involuntary biological functions in yourself or others, so no matter how much you practice your particular craft there will always be significant risk in this sort of activity.
Long Answer (an introduction to the unmitigated risks):
The primary risks of strangulation or choking not associated with gross physical damage are the restriction of blood or oxygen to the brain and heart, and related side-effects. Ischemia alone can cause stress to the heart, blood pressure spikes, and stroke, but the majority of the risk comes from the hypoxic and hypercapnic effects that follow ischemia.
Hypoxia can cause things like ectopic (premature) ventricular contraction (your heart "skips" a beat), and if that occurs at the wrong time in your sinus rhythm (during a normal t-wave, for instance) you could have a heart attack. No preamble, no warning signs, just sudden ventricular fibrillation caused by an unfortunate collision of two events, one of which you could have prevented from occurring. Every time a "PVC over T" occurs the likelihood of it repeating increases, and multiple PVC over T events in a chain will stop the heart. Even cardiologists can't always detect individual PVC over T events in the wild without the assistance of an EKG, so it is absurd to think that even with training you can tell by looking at someone's eyes, taking their pulse manually, watching their breathing, etc you are "informed" of their cardiovascular state beyond being able to see obvious signs of a heart attack or loss of consciousness. By the time these problems are observable, CPR isn't nearly as effective as you might imagine (about a 10% success rate, statistically). Incidentally, a person's overall cardiac health and fitness play a very small role in this risk; just like strong swimmers and divers can have a shallow-water blackout without warning, even athletes can have sudden fibrillation or tachycardia.
Hypoxia also causes metabolic acidosis by starving a glycolysis process in which glucose breaks down into pyruvate and creates a particularly useful little nucleotide called Adenosine-5-triphosphate, which is the transport mechanism for chemical energy within cells. Normally, pyruvate recombines with oxygen to produce more ATP, but if there isn't enough O2 to metabolize the pyruvate it turns into lactic acid instead, which being a form of metabolic acidosis decreases your blood pH.
Hypercapnia causes respiratory acidosis; basically, your body keeps CO2 in equilibrium with water as carbonic acid (H2O + CO2 -> H2CO3), which deteriorates into bicarbonate and an acidic hydrogen ion (HCO3- and H+). If breathing is restricted, hypercapnia (too much CO2) leads to a shitload of acidic hydrogen as your body attempts to balance things according to the above formula which, like metabolic acidosis caused by hypoxia, decreases your blood pH. If you hyperventilate, you'll instead get rid of too much CO2 at once and the process will re-balance in the other direction, raising your pH (respiratory alkalosis). Your bood pH is roughly 7.35 to 7.45 normally; the survivable range (for any mammal) is a pH of about 6.8 to 7.8. Anything outside of that range causes death at a cellular level very quickly, and since the impacted areas are a person's heart and brain, just a little taste of that problem is quickly fatal.
Assuming you do not experience one of the problems above, repeated cerebral hypoxic events will also likely cause vascular lesions that eventually result in a condition known as multi-infarct dementia, which is similar to Alzheimer's Disease.
Ironically, all things being equal a proper controlled strangulation is less risky than being knocked out.