You may not believe this (I didn't when I was told to do it) but it does indeed work. I have a SouthBend Heavy 10 with a 2 1/4 x 8 spindle. I have several chucks - buck 3 jaw, buck 6 jaw, skinner 4 jaw they all seem to get stuck once in a while no matter what I do to prevent it. (clean, oil, etc)
A couple years back I had my chuck stuck really tight. No amount of persuasion worked. I finally made an aluminum wrench to hold the spindle and bolted a 5 ft square tube to it. With a smile on my face and some wood blocks to prevent the chuck from turning, and some rosin on the spindle to keep the wrench from slipping, I proceeded to bend the 5 ft square tube (2 inch)! I was extremely disappointed and frustrated. My engineering expertise is not in the mechanical field. I called a buddy that is a registered PE and he laughed at me. He told me to put a 3 or 4 ft bar in the chuck (sideways) All I had was a giant screwdriver that I use as a pry bar. (about 30 inches long). He then asked if I had a 5 gallon bucket. I did. He had me hang the bucket on the end of the screwdriver (back gear engaged) and told me to fill the bucket with scrap! I had some paver bricks, a couple bags of lead shot (I load shotshells) and some other junk I put in the bucket. I left the shop and went in the house to eat dinner. About an hour later, I hear this loud noise coming from the garage. The bucket was on the floor, one of the pavers was broken, lead shot was everywhere, but the chuck was loose! It turns out that slow, steady pressure trumps the grunting and groaning and pulling on the wrench! My guess is that I had about 75 pounds (afraid to put any more in the Homer bucket), about 26 inches away from the lathe centerline. I have done this twice since that time and have refined the process to add a rope off the screwdriver to have the bucket hang only 5 or 6 inches off the floor. Works for me!