10-hp Yardman MTD Woodchipper

Wood Chipper.I would like to know the function of each in a 10 hp yardman,MTD woodchipper.The cutter blades,and the hammers. I know the cutter blades cut the wood, But what PULLES in the larger branches like 2 inch into the chute.the chipper that I have sometimes pulles in the branches and sometimes it dosen''t pull in the branches.Would it be that the blades need sharping?It dose make small 1/2 inch chips. Im mostly want to know what "pulles in the branches into the chipper.

Thank you

John