While I use the word, "D.I." in this article, Never, EVER, call your drill instructor a "D.I." Your drill instructor is referred to as "Drill Instructor [Rank][Name]."
Drill instructors are not supposed to use profanity, nor are they allowed to physically touch a recruit (other than for safety reasons, such as on the weapon’s range). So, how to they maintain discipline? In the other services, it may be push-ups, or possibly some running. In the Marine Corps, you get "quarter-decked."
Your three drill instructors work as an effective team. The senior D.I. gives most of the commands and orders. The "second hat," or "Heavy A," singles out those who seem to be having problems understanding simple shouted English and administers note-worthy tongue-lashings. To keep things interesting, the "Third Hat," administers the physical discipline, known officially as IPT (Incentive Physical Training), unofficially known as "quarter-decking."
IPT consists of prescribed exercises (a maximum of five minutes outside in "the pit," no maximum inside). Exercises one can expect if one is "quarter-decked" are: bends & thrusts, leg lifts, side lunges, mountain climbing, running in place, side straddle hops, and push-ups, done as fast as the D.I. can "encourage" you to. D.I.’s use a combination of individual and group IPTs to keep the platoon "on their toes."
During the "forming" portion of week one, you and your platoon won’t be able to do anything right, and you’ll be quarter-decked often. Some "jobs" can expect to be quarter-decked more than usual. Because of their relative high-visibility, the person chosen as platoon leader, as well as squad leaders, and those chosen to be "administrative assistants" to the Senior D.I. can expect more than their fair share of quarter-decking.
A Word About "Punishment"
One more word about discipline: It’s a whole lot easier to get into Marine boot camp than it is to get out of it. The Marines traditionally only fail about 15 percent of all recruits. D.I.’s are a stubborn lot, and while it’s possible to finally get thrown out, the way to discharge will be long and hard (simply refusing is not an option — that way lies court-martial).