Basically, the way we play tracker (mechanical action) organs or any other type of organs is the same on all instruments. It's just the possibility to control the speed of opening the valves under the pipes that is unique for tracker organs.
You see, in tracker organs there is a possibility to physically feel the breaking point when that valve starts to open.
Just hold the key with two fingers and start to depress it very slowly. At some point, you will start to hear this hissing sound - the beginning of the sound, actually.
If you depress the keys on such organs slowly and gently, you will hear the sound coming out of the pipes in a much gentler way.
If you want accents, especially in dramatic places, you have to depress the keys with a crisp motion of the finger. This must be done without using excess force, of course, still mezzo piano, without lifting your fingers off the keys. But this sudden motion will produce a different effect on the sound.
On electric, electronic, pneumatical, electro-pneumatical action organs this is not possible to achieve. This differentiation in touch is unique for mechanical action organs only.
A word of caution: since in the majority of organ music the attack and release of the keys are very important, when you have a soft and gentle touch make sure you are precise in your depression of the keys. In other words, in places where you have to press two or more keys at the same time, try to depress them exactly together.
The same can be said about the release of the keys. When you release a chord with several notes, make sure all the notes stop sounding at the same time.
Just try this out on your tracker organ today and you'll see what I mean.
By the way, do you want to learn my special powerful techniques which help me to master any piece of organ music up to 10 times faster? If so, download my free Organ Practice Guide.
Or if you want to learn to improvise in the style of Bach, I suggest you check out my free 9 day mini course in Keyboard Prelude Improvisation.