I got a very strong hit and it had almost no bruise. How is it possible to hit in this way? Is it a special technique?
Yes, it is possible (As you have seen demonstrated on yourself).
What causes a bruise is actually blood leaking into tissue from damaged vessels. The colors of the bruise are the blood cells dying and decaying in the tissues and being reabsorbed.
Even if you get really hard, if the angle or surface area of the strike is such that no blood vessels get damaged, then you won't get leakage, and hence no bruise. This tends to happen with flatter strikes that have a large surface area (Such as an instep round kick to the outside thigh), or strikes that hit an area with a relatively low number of blood vessels.
If you get hit with a more concentrated force in a smaller area, then the odds of you getting a bruise from it go up. It is not, however, a guarantee that you will or will not get a bruise.
Additionally, if you do get an internal bruise, the blood may not make its way up enough to be visible. It is still there, just not visible.