You must train against uncooperative and resisting opponents.
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A lot of martial arts techniques and applications just don’t work against a non-compliant resisting opponent. Especially so when applied in the ways that the vast majority of people try to apply it.
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How many times have you seen bunkai or partner drills with an opponents arm just hanging in mid-air waiting for you to complete your awesome technique? Or his opposite arm hovering in mid-air that in all probability would be punching your lights out, waiting for your to complete your other awesome technique?
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Most martial artists, traditional and modern, are in denial. They think that what they do will automatically translate into a ‘real’ situation.
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Locking or throwing a resisting opponent without some sort of ‘atemi’ (striking) is extraordinarily difficult (difficult enough that a lot of judoka will flat out tell you “standing locks don’t work”).
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There are people (very few of them) who can do this, but most of us mere mortals are not among them. That doesn’t mean you shouldn’t practice such techniques – they’re very useful in appropriate contexts for creating openings, unbalancing opponents, increasing the effectiveness of strikes, and neutralizing some attacks. Locking and throwing is a useful skill – it’s just not a magic formula.
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Size and strength always play a part, and locks and throws should not be your preferred option. Gichin Funakoshi said; “Never forget that the essence of karate is found in ending the fight with a single kick or strike. Great care must be taken, not to be defeated, by being overly concerned with applying a throw or lock.”
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Having a compliant partner is a must to learn a technique, and this should be followed by progressive resistance, or non compliance to add realism and build your skill. In training there will always be a point you cannot cross for safety of your training partner…..
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But remember. If your technique doesn’t consistently work with resistance and pressure testing, IT WON’T WORK. PERIOD.
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In a real physical self-defense situation, your opponent is going to both resist and be uncooperative. He may punch you, shove you, attempt to run away, or do something completely unexpected. He will NOT allow you to do what you want with him.
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Whatever system you train, once you have learned the technique, you must make sure your partner is both resisting and uncooperative. RESISTANCE IS NOT FUTILE. 
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