There are 2 things that happen when A hits B. 1: Some of the momentum is transferred from A to B, accelerating B. 2: Some of the momentum deforms B. The ratio between #1 and #2 depends on the surface area of contact and the speed difference between A and B. Let's ignore surface area for the moment.
Imagine standing next to a glacier that is slowly sliding down a mountain at a rate of about 10 cm a year. Because it is HUGE, it has a ridiculous amount of momentum. The speed is about 1.14e-8 km per hour. Let's say it is 1 km x 1 km x 100 m of ice, or 1.0e23 kgs. If you stand next to it, it will very, very, very slowly push you down the hill and you will not take any damage. Now imagine a truck weighing 5000 kg. To have the same momentum as the glacier, it will need to travel faster than the speed of light... So pick any ridiculous speed you want and imagine how much B will be deformed. Basically it will be vaporised before very much momentum is transferred.
When talking about damaging the head, what often happens is that the head is accelerated, causing the brain to hit the skull. You can think of your brain as a passenger in a car. If the car accelerates quickly, the passenger is pushed into the side. Same basic principal. If you want to cause this kind of damage (to knock someone out, for example), you want to strike relatively slowly, but with a lot of strength (force). This minimises the deformation and maximises the acceleration. On the other hand, if you want to deform the head (crack open the skull), you want to hit very quickly.
This is one of the reasons that boxing uses boxing gloves. It minimises broken bones, but at the potential expense of internal damage. The padding absorbs some of the impact, which effectively reduces the impact. If you are strong, you can punch "through" your target, accelerating it. This allows you to position your opponent and also knock them out. It also allows people with relatively poor striking form to hit without breaking their own bones (oddly, due to physics, if A strikes B, A is more likely to break).
Very quickly, I want to discuss the matter of bicycle helmets because I think it's kind of important to understand in real life. I'll explain in 2 dimensions because it is easier, but it applies equally in 3 dimensions. Let's say your head is 2 meters above the ground when you are riding your bike. If you fall over with nothing to slow you down, your head will be travelling about 22.5 km/h when it hits the pavement. This is why people routinely die from hitting their head on the pavement if someone knocks them out (important note: *never* get into a fight -- it's more dangerous than you think).
Now, imagine that you are riding forward on your bike at 40 km/h and you fall off your bike. At what speed does your head hit the ground? Answer: 22.5 km/h. Why? You are still going forward at 40 km/h, but in a *forward* direction. The speed in the *down* direction is still the same as if you weren't moving forward at all. The only difference is that pavement will pull all the skin off whatever is on the down side. Just to help the logic sink in, imagine falling off your bike and *just* before you hit the ground, you hit a street light. How fast do you hit the street light? 40 km/h. And then you hit the ground at 22.5 km/h...
Now, here's the thing. Bicycle helmets are tested by dropping them 2 meters onto an anvil. In other words, *it is tested at 22.5 km/h*. It will save your life if you fall off your bike at any speed *as long as you don't hit anything in the forward direction*. If you hit anything in the forward direction, you are pretty much toast. It is only meant to protect you from hitting the pavement. However, *most* accidents happen that way.
And just for some useless anecdata: When I was first dating my wife we went for a bike ride. I had bought a bike helmet for her, but she didn't want to wear it. Eventually she agreed to shut me up. About 30 km out, my wife managed to get her wheel caught in a grate. I watched as over she went, landing directly on her head. It split the helmet in two. We cycled home. She never gets on her bike without a helmet any more.