The 'hop' is due to pressure generated by axie expelling ingesta during regurgitation. The 'hop' per se does not affect digestion. It is like how you involuntarily 'close' your eyes when you sneeze. Some axies can also regurgitate without a visible 'hop'.
This varies axie to axie and on how sudden and drastic the water chemistry changes. It is not only salt baths at high concentrations that can cause this. Certain medications and even high levels of nitrogenous wastes in the water can all affect digestion rate. Some members here have experienced their axies regurgitating when in salt baths or medicated baths.
You are right though that a lot of the time, my explanations are drawn from theory although i have witnessed them several times personally.
Here are some good veterinary texts and journals. If i can obtain PDF files, i would link it here, otherwise, your library may have these resources.
Clayton, L.A. (2005). Amphibian gastroenterology. Veterinary Clinics of North America, Exotic Animal Practice 8(2): 227-245.
Hadfield, C.A. and B.R. Whitaker (2005). Amphibian emergency medicine and care. Seminars in Avian and Exotic Pet Medicine 14(2): 79-89.
Wright, K. (2006). Important clinical aspects of amphibian physiology. Small Animal and Exotics Proceedings of the North American Veterinary Conference. 20: 1686-1688.
The following paper discussesgill biopsies i mentioned about in another post to detect gill pathology when the axie is subjected to adverse water conditions ie temperatures.
Pessier, A.P. (2007). Cytologic diagnosis of disease in amphibians. Veterinary Clinics of North America, Exotic Animal Practice 10(1): 187-206.