Resistance in Aikido
If you spend a little bit of time on-line looking at what other martial arts practitioners have to say about aikido, one of the thing you note is that people with little or no experience whatsoever about aikido still have plenty to say about it.
The most common comments is that aikido has no sparring and doesn't train against opponents offering resistance.
Let me first take the latter point. What others assert here is that in aikido dojos people don't train to execute a technique against someone actively resisting it.
The is a little bit true, a little bit false, and a whole lot completely missing the point.
What people see as training against no resistance is the stage when beginners are learning a technique and need to understand its shape. At this stage the uke, the attacker, who ideally will be a more senior student will indeed sometimes help the other person complete the technique.
This is not dissimilar to early training in other martial arts where people punch thin air, or cut it with bokken.
However, as the student progresses you can expect the uke to be much less of a push over, literally and metaphorically - at least in most dojos. People don't just fall over when the technique is wrong, there may indeed be some resistance. There will be a touch here and there that could have been a counter strike. How much of this resistance there should be is a point of real controversy. It is possible to find dojos who think it should always be none, and others who think it should be immense and unrelenting.
In the main there is a recognition that it does not demonstrate skill to prevent a technique that you know is coming, and so it's poor form to do so for its own sake when people are first learning a technique.
For some people this demonstrates the inefficacy of aikido, but it's no different than suggesting that a person in another art, knowing that a kick to the head is coming, can easily avoid or counter that.
The Way of Harmony
But the main point here is what ai ki do means - the way of harmony with ki of your attacker, or the energy of your attacker. In other words, when an opponent resists, they resist a specific thing. The whole point of aikido is not to force your technique against your opponent's energy. Yes, you may be able to force it on. But that is not aikido. There are also tricks of the trade that can make a technique work even when it's probably stupid to apply it, but these should be reserved for moments when you find yourself in the equivalent of having your heels on a cliff edge with tigers below.
So what should an aikidoka do, when they begin technique A and their opponent resists it? Pretty much anything other than A. When you watch someone do aikido badly, you can see this tension all the time, the nage, the defender, has a plan to do technique A. The painful bit is that if A is not possible, the longer it takes nage to work this out, and move on, the movement between attacker and defender jams up, it is actually a point of real vulnerability, not strength for the nage.
So aikido is about learning that the instant A becomes untenable, you let go of it, and move to B, C, and so on. The more someone resists a specific thing, the more vulnerable they become to its opposite.
So coming back to the lack of "sparring". This is actually also untrue, this kind of practice does occur in many dojos - it's usually a different kind of sparring - and it's valuable if controlled, but real aikido happens all the time when the initial technique goes wrong, the challenge is to realise that trying to push through resistance is completely the wrong approach.