Sparring teaches about distance, timing, reading the attack with visual clues as opposed to tactile clues. Distance fighting can involve hit and run tactics, which are not a part of normal chi sau although it can be. If someone is swinging a bat at you or an Escrima stick or trying to cut you with a knife then you develop a certain set of distancing, timing and entry skills and pick up on a lot of clues that are different from Chi sau training.
But Chi sau means different things to different people. For some it is just sensitivity training while for others it is a real fight just as much as some people consider BJJ mat wrestling a real fight. In one lineage I trained at, chi sau was everything including distance. That means we started from chi sau, then started from an inch distance, then 6 inches, then a foot, then three feet. The idea was to not throw away all the skills developed in contact chi sau but to change only what needed changing when dealing with distance. So everything was looked at like a continuous spectrum from distance, to chi sau to ground grappling. It was a continuous thing.
Distance fighting taught one how to make the contact. If you are faced with a Capoeira fighter who spends most of his life upside down, then making contact is not easy. Making contact with a hit and run fighter is also not easy. There are all kinds of funny fighters. Making contact with a football tackle is easy on the other hand, because he does it for you. But another set of problems arises with that. With protection on, you might do a simple leg life and block your training partner’s roundhouse kick and feel fine. But without the protection you might not like the pain and instead develop better timing, better positioning and better footwork skills to avoid that pain. So padded training isn’t always the best but I understand the old argument that Thais and Boxers do it and they are good.
I have seen lots of cases where people go from good Chi sau to being absolutely helpless in the distanced case. It only means they haven’t practiced sparring. For a real fight these people would have to manage to get the guy to play their game. In the stories where chi sau guys have won in real fights, it’s because the opponent started in chi sau range or the chi sau guy just used his wits was careful and found a time to close. In a general ring fight they would lose because closing without getting hit is the big problem.
In Chi sau everyone has a chance including a small lady against a much heavier guy. In all out sparring a small lady doesn’t have much chance against a 250-pound guy and neither do a lot of small guys unless they are incredibly fast. I tried sparring against a SanDa champion who had 10 years of SanDa. But I outweighed him by 30 or 40 pounds and had longer reach and so he couldn’t do much of anything. If he kicked, I just crashed into his kicks with my kicks. His handwork was nothing compared to Wing Chun handwork. He was stiff and tense (relatively speaking) which might be ok if he outweighed me but he didn’t. So it was easy to push him, pull him, jerk him and do all kinds of stuff to him. However when he sparred with an equal size opponent he looked very good, could hit well, could throw, sweep, kick hard and do all the good stuff.
So in a way his training may have been dry land training as well in that he didn’t apply it against the heavyweights. Perhaps it can be made to work but perhaps not.
To me sparring against equal opponents doesn’t do that much. But sparring against a heavyweight relative to your size teaches you something because you can’t afford to take one of their hits. With equal opponents maybe you can go all out 100% whacking the hell out of each other but try that against a guy 40 pounds heavier who is more skillful than you and you will have some major health problems happening. If he fights you at 50% then it’s not the same as fighting a heavier guy at all.
In martial arts you should assume the guy is faster, stronger and heavier than you. If you can hit someone like that then you have a real martial art, you have real skill.
I wonder if boxers and Thai boxers can handle people who outweigh them by more than 40 pounds any better than martial artists can. It seems the whole premise of martial art is that it is possible for a smaller individual to defeat a larger one. If the large one can’t fight or has less skill then of course it’s easy.
In my opinion, chi sau teaches you what to do when contact has been made. Sparring teaches you how to make the contact. Some people have developed training procedures that almost seamlessly go from one environment into the other.