Aikido practice is very cooperative. We have both partners who know what end result is trying to be realized. They know what the starting conditions are (e.g., a grab, a strike). Even then, what exactly is supposed to happen is fuzzy, especially during the technique. Mostly people tend to find some “safe” area to agree to work within. However, the myriad understandings of what is supposed to happen means that there are just as many ways that one might think to take it out of the “safe” zone and be more sincere or intense.
There is the ‘uke’ doing the initial attack strongly or fast in a way that eliminates the possibility for responsiveness. There is also the ‘uke’ breaking contact and/or decreasing engagement during the technique. The ‘uke’ person does various things to thwart the end result(s) and ‘tori’ is expected to a) continue to try to execute a technique or b) execute various evasive actions and maintain some control of self and situation. Supposedly this way of doing ‘uke’ makes the practice more intense because ‘tori’ must deal with various unpredictable actions by ‘uke’ and is under more pressure to be in complete control of ‘uke’.
In very small amounts, I believe this kind of practice may be eye-opening. It may show ‘tori’ how little control he has and how much he normally depends on uke’s cooperation. The downside is, the interaction between the partners is inherently decreased. This makes for a shallower practice. Furthermore, establishing some form of connection and consequently more interaction is something that the stronger and more aggressive person can do. That is, that person can grab on to the ‘uke’ or counterattack more. This makes the practice better suited to certain physiques and temperaments. Another piece that makes this “intense” is often speed. Unfortunately, speed is not truly a big problem and more meaningless still when the action is more-or-less predecided. In all of the above cases, the ‘uke’ is mostly taking away any need for ‘tori’ to do anything. It only looks “intense” when ‘tori’ tries and continues to try to do anything, as it isn’t appropriate to the situation anymore. And from what I’ve seen, ‘uke”s priority seems to be more about self-preservation than hitting ‘tori’.
Personally, I would consider two kinds of practice to be “intense”.
First is a variation on the breaking contact/decreasing engagement scenario above. The crucial caveat would be for ‘uke’ to be very discriminating about when it is opportune to do so. Oftentimes, there are many instances in which a) ‘uke’ is just running away or b) ‘uke’ is oblivious to their own openings, in part thanks to ‘tori’ being busy trying to execute a technique and not hitting ‘uke’. I think that including the possibility of ‘uke’ not cooperating at any point during the technique increases the intensity and necessary concentration of both partners.
Second is practice in which you give and receive from each other all your power. As ‘uke’, you place yourself where ‘tori’ can throw your center as optimally hard as he can while absorbing his power with your center as much as you can. When attacking/during the technique, ‘uke’ maintains some muscular tension all throughout while always moving in accordance with ‘tori’, not in some other direction nor too late or early. This practice is not restricted to both partners being of similar physical build and mutually interested in youthfully vigorous practice.) That is, there are adjustments made to be appropriate for each partner’s limitations of power and endurance.
At this time, I’m mostly inclined to view “serious” practice as more appealing than “intense” or extraordinarily “sincere” practice. I try to make this the norm as best as I can. While working to keep the engagement continuous, respecting and accepting the natural ebbs and flows, not forgiving ‘uke’ opportunity to break connection with any advantage, and being fully “available” to any kind of adaptation in the moment.