hello,
i know there’s a visual keyframe which is like the ‘apply visual transform’ . The problem, it’s static.
Is it possible to have that keyframe doing the same but dynamicaly ? In other words, it will do the “apply visual transform” each time the time cursor runs over it . In consequences, it would be like “resetting” the effect of a constraint by applying it.
I hope it sounds clear 
You might be looking for the idea of baking visual transforms. Look at the “bake action” operation. This lets you remap existing keyframes on constrained bones/objects to keyframes that represent visual transforms. Look at the manual, look at the settings.
One example of a time you’d use this is if trying to create an animation for use in a different engine where you don’t have access to all of Blender’s constraints.
As far as, “when the cursor runs over it,” uhh, I’m not sure why you’d want that. Maybe just to make it less irritating to rotate an object with a shrinkwrap constraint or something? Many constraints have a “for transforms” checkbox that should work that way, but it doesn’t work very well in my experience. I remember it working better (not perfectly) previous to 2.8…
If you brought up the specific situation that you’re unhappy with, that you think this would solve, people could probably offer better advice.
yes, i know about baking actions. But it’s a static solution : it’s typically applying things as they are setted on the present time.
i cannot say it with more simple words. I want to create an action strip in which a keyframe will do the same job as an '“apply visual transform” . So if i re-use this action somewhere else, later or on another object, this special keyframe will do exactely the same : apply the visual transform just like a “cntr+A” would. So it’s dynamic.
Okay. What problem are you encountering that you think that would help with?
easy switch between IK and FK
So you wish that you could just move an IK target, and it would apply the IK as a visual transform to affected bones?
If that’s the case, you should look at auto IK (in pose mode, with bone selected, sidebar/tool/pose options) which works exactly like that.
Just be aware that the important difference between IK and FK is the interpolation-- interpolating between two snapped FK keyframes is very different than interpolating between two real IK keyframes.