The best I can explain is this… In Spring with increasing daylight and progressively warmer temperatures the plant begins to move resources. The carbohydrates, water, nutrients and hormones present in roots and vascular tissue begins to accumulate in buds on trunk and branches. At this point the roots serve to anchor the plant and transport water and nutrients from soil or potting medium to the branch tips where the buds are located to fuel their growth. Roots also grow at this time as well, but primarily its going to be foliar growth.
So, when doing spring repotting I feel the key is to trim roots back, only removing overly coarse(thick) roots. The fine roots are the ones delivering nutrients and water to the rest of the plant. The thick roots have function as well, storage and stability to simplify thses are usually interior roots and nebari type roots.
So for me I do only as much is required to keep the plant strong and healthy. Disturb only what needs to be removed, use appropriate mix for the species.
I think most failures during potting/repotting is inconsiderate and over aggressive root work, far too frequent repotting and the number one killer is inappropriate aftercare. Aftercare including watering, sunlight and protection. Aftercare is plain complicated.
Tips for you watch Ryan’s repotting videos. Learn his method. Find nebari, reduce starting from greatest limitation (top, bottom, exterior), know what your removing, aeration and proper chopsticking.
Hope you find this helpful, good luck with bonsai.