I am gonna split some hairs here, mostly because the OP is talking about energy work and not joint lock technique...
First, I do not believe pain is a requirement of expressing aiki. That is, I believe for the purpose of energy work, aiki can be expressed without intending pain; there is some responsibility for both partners to manage their bodies to minimize discomfort. Kansetsu waza as a jujutsu is designed to physically manipulate body parts for the purpose of creating discomfort as a tactic of control. But now things get murky because at some point we end up getting closer to poor jujutsu and farther from aikido. And if you think you kansetsu waza is up to par, just go roll with your neighborhood judo or BJJ club. Maybe you're in good shape, maybe not.
But we don't do jujutsu, so what makes our stuff different? Aiki, right? So how to you 'aiki" kansetsu waza? What if the manipulation that locks the body is not localized in a joint? What if you could manipulate your muscles and tissue around
your joints to insulate and protect them? You could use your muscles and tissues to pull the stress out of a joint? Or, manipulate your partner's muscle and tissue to bind or immobilize a joint... Not pain compliance, but restriction of movement.
At some point, the idea of me "giving" you my arm to bend becomes a farce. In MMA and other sport fighting, you don't see much success from small joint manipulation because most competent fighters will defend their joints. The bigger joints can weaken under stress and you see success with arm bars, triangles, and shoulder locks once the defender is fatigues or fails to see a threat. But, that means you are fighting until you get to slip in a move. Not very "aikido", even if it is fight science 101.
Second, go with the weirdest answer. The best aiki people I work with really could care less what I do. You decide what to do with the energy in you body; whether that comes from your cultivation or as a gift from your partner, its your energy to manipulate. At some point you care where you put the energy because that is what designs technique. When someone puts excessive energy into you, you want time and space to remove that energy from your body. I have seen people take punches where you can see the exit force bruise because there is no time to disperse the energy. It's just physics, but it's freaky.
There are tons of stories from the old students describing ukemi for O Sensei as falling without control, being struck by lightning, instantaneously being put on the ground. In judo, they called this dashing. Daito Ryu students told similar stories about Sagawa Sensei. None of them said ,"O Sensei twisted my wrsist so hard I had to tap." This is important for me because it tells me the feelings the students remembered were not pain, but instant control.
The ground is the closest point of impact. As an hypotenuse of the right triangle of the ground and me, the closer my partner is to my feet the shorter the distance traveled to impact. Throwing someway "away" makes for more time and space for you partner to manage the energy in her body. Aikido ukemi is somewhat unique because most other fight systems don't deliberately provide for the opportunity to recover from an attack. Originally, I believe this was probably done as a safety mechanism to let nage express aiki in practice; now, it's become something of a stylized falling system.
Here's some videos:
Sunadomari Sensei talking about power, taking power from your partner and then expressing power in your movement.
This is Chen Xiowang showing push hands. It's not quite our thing, but a good demonstration of power expressed horizontally into a partner
In both videos, you can see horizontal expression of power giving partners time and space to recover. Now just imagine if you angle that power into the ground and don't give your partner time to recover...
Oh wait, here's a vid of Shioda sensei. You can see the vertical alignment of uke in many of sensei's throws; these throws are brutal - just a little more over vertical and these uke would be stacking their bodies onto their neck