@Hobbes's answer calls attention to Emily Lakdawalla's book The Design and Engineering of Curiosity : How the Mars Rover Performs Its Job which led me to reading excerpts in several Planetary Society blogposts including Book Update: The Design and Engineering of Curiosity which includes the following image(s) and description.
At the bottom there is something called a secondary thwack arm.
Question: What is a secondary thwack arm, and what gets thwacked? I certainly hope it's not the bee trap! (shown in the linked original image link below)
("bonus points" for an image of the primary thwack arm.)
Original image but answer shows it as well.
Parts of the CUriosity Chirma 1-millimeter Sieve Pathway
A figure from the forthcoming book The Design and Engineering of Curiosity: How the Mars Rover Performs its Job, by Emily Lakdawalla, illustrating the parts of Curiosity's sample handling mechanism that sift and portion out powdered samples of Martian material using a sieve with 1-millimeter holes.
NASA / JPL / MSSS / Emily Lakdawalla