When you should use whey hydrolyzate instead of whey concentrate
If you can use shakes with a substantial amount of protein powder, then a shake with relatively cheap whey concentrate works just as well as a shake with the expensive whey hydrolyzate. But if, for whatever reason, you can only use shakes with little protein, then you should probably use a shake with whey hydrolyzate. Researchers from the Japanese dairy group Meiji discovered this. [meiji.com]Study
The research that the Japanese published in Nutrition and Metabolism is an animal study. The researchers got rats to swim for two hours in a series of experiments, and gave them proteins immediately afterwards.

The researchers then determined the production of new muscle tissue in the triceps of the animals. If you want to know more precisely: the researchers looked at the fractional synthetic rate or FSR.
Animal study 1

In a first experiment, the researchers gave their rats 2 grams of whey concentrate per kilo of body weight. They discovered that the fractional synthetic rate peaked one hour after administration.

Animal study 2
In a second series of experiments, after the swimming session the researchers gave their rats water, 0.5 or 2 grams of whey concentrate per kilo of body weight, or 0.5 or 2 grams of whey hydrolyzate per kilo of body weight. The rats that were given water functioned as a control group.

The difference between whey hydrolyzate and whey concentrate is that the proteins in whey hydrolyzate are cut into pieces. The size of these pieces is different for each product.
When the researchers studied the triceps of the rats after 1 hour, ie at the time that the fractional synthetic rate should be maximal, they saw that at the relatively high dose of 2 grams the fractional synthetic rate in the whey concentrate group was about the same as with the whey hydrolyzate group.

However, it was different for the rats that received the lower dose of 0.5 grams. At this dose, whey hydrolyzate was the better muscle builder.
The researchers do not know why whey hydrolyzate has a more anabolic effect with a relatively low intake than whey concentrate. According to a popular theory, whey hydrolyzate leads to a rapid increase in the concentration of amino acids in the blood. This increase should stimulates anabolic processes. The Japanese monitored the blood of their test animals, and saw that this theory was incorrect.

The researchers suspect that whey hydrolyzate contains peptides with an anabolic effect that are present to a far lesser extent in whey concentrate. You can read more about those dipeptides here.
"These results demonstrate that ingestion of whey protein hydrolyzate was associated with greater post-exercise muscle protein synthesis compared with intact whey protein [...] at lower doses," the researchers conclude.

"Although intact whey protein is consumed worldwide and it is well known that intact whey protein intake is highly effective for stimulating muscle protein synthesis, whey protein hydrolyzate may be an alternative nutrition strategy to improve protein synthesis and impact muscle mass maintenance."
Nutr Metab (Lond). 2019;16:90.