Fruitnuts explanation was excellent.
I'm learning to chemical thin about 800 dwarf apple trees - roughly 20 varieties. Some varieties thin easy and some like Fuji do not. Weather can greatly impact the results, so the rates of the chemicals need to be adjusted based on last years results as well as the actual weather several days before and predicted weather several days after the thinning spray. Also the size of the fruit at the time of the thinning makes a big difference.
Check out my reply earlier in this thread for a reference to common thinning chemicals in the SouthEast Apple Production Manual. I use the NEWA thinning model from Cornell to help adjust my rates based on weather. Sunny= less thinning. Cool= less thinning.
An article from Penn State explains it well:
I follow up with hand thinning after the chemical thin. With more practice and some luck, I hope to improve the results of my chemical thinning so I can reduce the hand thinning.