When the technique is right and the only remaining thing to do is application of brute force. But again, it’s assuming that both you and your partner/opponent are going at 80% which is my understanding of sparring or randori.
Let me explain this with the armbar from mount example. If my opponent is setting up an armbar, I’ll try different ways to escape and if he’s pinning me down tightly closing all avenues of escape so there’s no other option but to defend with, let’s say a cross lapel hold. So if he starts prying my fingers and I feel my grip loosening, I’ll resists until a reasonable level of effort and that is to say 80% after which if I’m see it’s a matter of five to ten seconds of his grinding, I’m letting the armbar, thus avoiding my nightmare scenario where he’s almost completely extending my arm while I’m (barely) resisting purely on the strength of my biceps.
That’s an injury waiting for happen and pointless dick-measuring contest outside of competition. Training smart helps you extend your training lifespan and improves your technique.
That’s the philosophical difference in approach to training - are you and your fellow team/club members trying to get better so that you can give your absolute best on the mat in competition when it really matters or are you the proverbial weekend warrior that shows up every class to beat and get himself beat up.
This is a nice summary I found on line:
According to Dr. Kano, there are three tools for instruction or practice: Kata, Randori, and Shiai. At the highest level, here’s what they mean:
• Kata: practice of forms; predetermined movements
• Randori: adversarial, free-moving practice
• Shiai: competition, tournament (I’ve posted before about the value of competition.)
I’m not going to talk about Kata here (though it bears discussion), but I do want to talk about the difference between Shiai and Randori because they’re sometimes confused.
Randori is practice. To “win” in Randori, you just need to learn something new or get better or help your partner learn. Thus, you can still win if you get thrown 30 times. The way you “lose” in Randori is that you don’t do any of those things and/or you hurt yourself. You are generally not putting your heart and soul into putting your partners back on the mat. Given the principle of "Jita Kyoei” (mutual benefit), you are trying both to help yourself as well as help your partner. That doesn’t mean that there’s no struggle, but it does mean that if a 200 pound black belt is going against a 150 pound green belt, the 200 pounder shouldn’t be making it as difficult for the green belt as he is capable. Maybe the 200 pounder will decide to focus more on foot sweeps, or he will give the green belt many looks at the same attack (even if that attack puts the green belt on his butt every time)…
Shiai, on the other hand, is a competition. You “win” in competition by winning. You put the other guy on his back, you pin him, you make him tap, you get more points, or he gets disqualified (though I’m not a fan of people who try to get the other guy disqualified). If, for whatever reason, the 200 pounder in the 150 pounder mentioned above were in the same division, and they found themselves facing off against each other, the 200 pounder’s job is to put the smaller man on his back with force and control. As I mentioned in the article I linked to above, both Randori and Shiai are about field testing your Judo. Shiai, though, is intended to be a more extreme environment, where the other guy is trying as hard as he can to put you on your back, and you’re seeing how your Judo works in that situation. Jita Kyoei still applies, but in this instance the benefit that you are providing your opponent is a sincere attack.
Randori has partners, Shiai has opponents.
One’s first Shiai is often an eye-opening experience. You get hit. You get ground up. You get mauled. You get jostled like you’ve never been jostled before. It may not be beautiful judo, but that isn’t to say that that’s not what your opponent is supposed to be doing. You see, they’re giving you the opportunity to test your judo against somebody who’s mauling, hitting, and jostling you. That’s a useful skill. Just be ready for it.
Also, here’s a short clip of my favorite judoka Flavio Canto rolling in BJJ to illustrate my point - look how smoothly he’s transitioning and giving up positions without ever resorting to grinding.