In my steady-state drawing of venous stenosis I omitted the fact that the heart's pressure is not steady but repetitive, and sometimes may go negative, reversing flow. Being sometimes negative or even just going down to zero pressure makes it sensitive to events like coughing, desalva, breath holding, or a punch in the chest. In a mathematical description, having a fundamental frequency means the system the heart beats into may add to its harmonics, affecting the pulse's waveshape, occasionally or even continuously. That is what Dr. Tucker describes in his YouTube animations.
The valve is supposed to prevent these excursions into negative territory by closing at the right time. That will work if its flap(s) remain free, and it does not become too sticky. It can also fail by being too loose, allowing pressure to overcome it's shut-off, or it's timing. It can also fail altogether by being 'blown through', rendering it of no further use. That is more likely in a leg vein, but gravity is a continuous stress on all veins, as we age.
How do people who have no jugular valves avoid the problem? Well, gravity is a significant component when we are upright, and the likelihood of strong standing-wave-type effects when we are supine is less, so unless a strong disturbance happens like a cough, valsalva, or a blow, there may be no problem when the heart is not beating hard. These events may cause jugular reflex when supine, if absent valves allow it in.
If we are vertical those who don't have valves may have learned that being upside down can make them very ill. This same problem can happen to those who have a stenosis, and significant collateral drainage, because the new growth will not develop new valves. My brother has only one original jugular, and he gets very very dizzy and sick on swings and rides, car-sick, etc. It is usually a good thing to have valves because they do protect the brain, and in the case of the legs, make sure all the blood stays in circulation.
What if the system were steady-state? If the heart were more like a rotary motor-driven pump? Could there still be steady-state reflux? I think evolution would have made sure it didn't happen, or that it had seldom had ill effects. But evolution can not protect individuals from late-onset problems because it only works to make the average person reach breeding age. So I think a) there can be occasional reflux and b) reflux can become pulsatile in older individuals. Keep your valves as long as you can, and when you are old, Father William, pray, don't stand on your head.
"Try - Just A Little Bit Harder" - Janis Joplin
CCSVI procedure Albany Aug 2010
'MS' is over - if you want it
Patients sans/without patience