Oh yeah I recall that some of the items in 3 had a +% chance for dismemberment but I think it was on killing blows only. In monster hunter you can get dismemberments that rarely actually changed the fight but gave rarer mats.
The monster in the trailer is assumably a Kikimora, which are relatively common, but hard to kill and therefore feared.
They have a very hard chitinous shell and are agile, but if you manage to slice up the legs, you can get closer to its' weakpoints and therefore kill it.