I had to look these up. "Lettrism" is a 1940s French artistic movement, closely associated with an artist named Isou, and not familiar to people who are not deeply interested in art and art history. "Amplic" and "chiselling" are technical terms invented by Isou himself. They are specific to discussions of this art movement. You are not going to find much help in general language books or ordinary dictionaries. I suggest the Wikipedia article on Lettrism, and the section on "the Amplic and the Chiselling phases," as a starting point.
"Chiselling" (spelled with two L's in the UK, and one L, "chiseling," in the US) means "to use a chisel."
A chisel is a cutting tool. There's a picture here:
A chisel is used for rough shaping. A woodcarver might use a chisel first to get a rough shape into the wood quickly. He then might use other tools to smooth it. Someone with a very rugged, irregular-looking, rough face can be said to have "chiseled" features.
Isou had some grand theory about the way in which any art form develops. "Amplic" and "chiseling" refer to different stages in any art form. The Wikipedia article says the amplic phase ends when the art form is fully developed and finished, and there is no more to be done. "This then inaugurated a chiselling phase in the art. Whereas the form had formerly been used as a tool to express things outside its own domain—events, feelings, etc.--it would then turn in on itself and become, perhaps only implicitly, its own subject matter." Because this sounds like pretentious nonsense to me, I can't explain it!