Ice is very similar to concrete pavers; it's both rigid (i.e. requires a lot of force to deflect), but brittle (i.e. takes very little deflection to break).

Understandable.

Still, nothing will detract from the visceral wonder that I could never forget after seeing, in person, an extremely high ranked karateka (i.e. second in the world, in his style) chop a seemingly solid block of ice over a foot thick.

I suspect that, while there might be "a trick" behind these things, extreme achievements in that field are still rare and accomplished acts, as they are in any. The wonderment need not be fully extinguished.

Oh, it's absolutely hard, but like many things the apparent difficulty and actual difficulty are only loosely related, and good showmanship (perhaps a more polite term for "trick") will involve maximizing the apparent difficulty while keeping the actual difficulty manageable.

You see similar things with e.g. juggling where entertainers will do a lot of things that make it look harder. This doesn't mean that it's not difficult to juggle 7 objects.

I heard that they sometimes the ice is broken with a hammer and refrozen, so that it has an invisible weakness.

Sometimes wooden boards are baked to make them weaker.

Only sometimes though.

I think that people might be using "trick" in two different senses.

There's a "trick" to legitimate breaking in that the practitioner is choosing materials and techniques that lend themselves to breaking, but that doesn't mean that the actual break is faked.

On the other hand, it's also possible to fake a break. For example, you can cut a piece of ice in half, put some water on each end of the cut, and stick them back together in the freezer so that the ice appears to be solid but actually has a weak spot.

There can also be an element of showmanship. Even if a martial artist has done a break dozens of times, during a demo they might make a show of having to hype themselves up for the break, tensing their muscles with a dramatic hiss of their breath, etc.