It seems the material turns hard in the instant it's struck, and distributes the force of the impact over a larger area. I've heard of such things for a while, and it was imagined in the famous SF book Ringworld from 1970, if I recall right.
This article says:
These materials, also know generically as “rate-dependent materials,” work by having their molecules freeze in place when struck hard, but are pliant when moved gently — just like water, which is a rate-dependent material of a sort. If you lower yourself into a bath there is little resistance. But slap the water hard with the flat of your hand and it will leave your palm stinging.
I think the latter is just platypus, though. With water it's just enertia, water is dense and can't get out of the way fast, so it feels hard to hit fast. (A speedboater said he'd rather fall of his motorcycle than his boat.)