Tamid, Chapter 6, Mishnah 2. This chapter follows the Kohanim into the Heichal for the final stages of the morning service. The previous mishnah dealt with the lamps of the Menorah. Now that the lamps have been attended to, the Mishnah turns to the coals that were carried inside in preparation for the Ketoret.
"Mi shezacha b'machtah": earlier in the Masechet we learned about the lotteries that distributed the various avodot among the Kohanim, and one of them awarded a Kohen the machtah, the pan in which the coals for the Ketoret were brought. It fell to him to gather the glowing coals and carry them, in the golden pan, from outside into the Heichal.
The Mishnah continues: "Tzavar et hagechalim," he piled the coals up, "al gabei hamizbeach," upon the inner Mizbeach, the golden one that stood within the Heichal itself.
"V'ridedan b'shulei hamachtah": and he then spread those coals out flat, pressing them down with the underside of the shovel. The reason is beautifully practical. The Ketoret was about to be placed upon these coals, and a heaped, uneven pile would have let the spices slide away. A level bed of coals held the Ketoret in place.
"V'yishtachava v'yatza": following the pattern of the Kohanim who had served before him, he bowed down and left the Heichal, taking the now empty shovel out with him.