Rajani can you share an image that has been de-identified so that we can look at it together?
In which segments did you witness this phenomenon?
I have some obvious questions about your technique: was the patient upright/standing? If not how steep was the patient inclined?
How do you augment flow? With hand compressions or with an automatic cuff inflator?
Prolonged reflux is never normal. Valsalva increases abdominal pressure which should reduce or stop spontaneous venous flow towards the heart. Distal augmentation will empty some of the blood from the veins so that there is capacity for refluxing blood to flow in a retrograde direction until the valves close or until the vein reaches capacitance.
If reflux is severe and the patient's legs are lower than their heart then the veins may expand until they have reached maximum capacitance. Then a valsalva may stop or reduce flow but not produce retrograde flow because, not only is there no place for the blood to go (you can't put water into a bucket that is full of water), but there is also more blood coming through the capillary bed from the arteries. To paraphrase Newman on Seinfeld: " the mail never stops. It just keeps coming and coming and coming,"
One possibility is that when the valsalva comes to and end, the flow reverses to follow a pathway that has less resistance. That would imply that the outflow connected to the refluxing vein has a proximal obstruction.
Again, without knowing your technique I'm must grasping at possibilities.
As for forward flow during valsalva, that to me suggests that there is a signficantly elevated venous pressure. The most likely explanation is failure of the right side of the heart as Mina hinted in her reply. There could also be a flow obstruction proximal to where you are samplling (iliac vein) which would elevate the venous pressure distal to that (could be a DVT, extrinsic compression by a mass or just the weight of the bowel). If the patient is obese try to reposition them on their hip to shift the weight of their abdomen off of the iliac vein on the side you are evaluating. There may be a combination of factors.
One other thing I will mention is that if the patient is inclined but not standing and if that patient has right heart failure (or a venous obstruction), the elevated venous pressure my be close to the valsalva pressure and the valves may simply float open aiding reflux. Not because of disease but because the pressure gradient is so small. Standing the patient up or tipping them quite steep along with a vigorous augmentation emptying of the veins may solve this. This is why automatic cuff inflators are so much better than bare hands.
Much to consider. I hope you can share a picture.
Steve