In my understanding they are the same behaviour with different terminology. I hear them used interchangeably.
Edit: Free ascent I hear from those with military backgrounds and a CESA is a recreational scuba term. Upon reflection I suspect there are differences as I've never been trained for a free ascent.
Free ascents are without a regulator in your mouth and sometimes with Scuba gear jettisoned. CESA (Controlled Emergency Swimming Ascent) keeps the regulator in your mouth to take advantage of any residual gas in your cylinders that becomes available as ambient pressure reduces. Submarine escape uses very buoyant ascents and often a breathing device, starting in the US with the
Momsen Lung and later the
Steinke Hood. There may be something newer now.
I prefer doing free ascents because I feel that the regulator makes it less natural to keep my airway open. After a lot of practice I found that I can position my neck and jaw so that it is very hard to close my airway. One of the divers I worked with taught buoyant ascents at the Submarine Escape Tower in New London and helped me refine my technique. Rather than overtly blowing air out I barely exhale and purse my lips to keep the water out and make sure that there is almost no backpressure.
Obviously, this can be fatal if you screw up so test carefully and progress slowly. However mastering it will increase your survivability no matter how much your gear fails. I didn’t need it since I had been doing them for about 9 years by then but he suggested sitting in a pool holding your breath and assume your "position"... looking up, jaw thrust forward, pursed lips. Air will escape when you find your sweet spot and gently pushing on your diaphragm will push air out faster.
You can force air out faster than necessary, which isn’t a big problem in itself since the expanding air will catch up... except that you may naturally try to close your airway and embolise yourself. That is true on a free ascent or CESA.
Start shallow and overtly exhale until you get the feel of it and SLOWLY learn to exhale less aggressively until keeping the airway open is natural and reflexive. Keep your regulator in hand in case you over-exhale and get uncomfortable. You will also learn the feel of moderate lung inflation and allowing gas to escape through your lips faster as you get shallower. It sounds a lot harder than it really is. I did one from 180' once because I was doing a lot of diving at that depth and wanted to make sure my ultimate back-up system worked.
Edit: I forgot to mention an important factor:
Remember that you won't pass out from low Oxygen because of the elevated ppO
2... like 2x at 33', 3x at 66', etc. There is way more Oxygen in your body than you will need to reach sunshine even if you have only been on the bottom for a few minutes. You will also be expelling CO
2 so can comfortably do a free ascent or CESA much longer than you can hold your breath at the same activity level.