Powerlifting is focused around 3 movements, which are the barbell squat, barbell bench press, and barbell deadlift. And as a competitive sport, powerlifting is where athletes compete to attain the best single rep max squat, bench press, and deadlift. In competition, each movement. Below is a breakdown of the competition standards of these 3 lifts, as well as accompanying videos from Barbell Brigade that do a great job of overviewing these standards as well.
Commands: Once you unrack the weight, which you can do a self unrack or have a handoff, you will be given the start command, which all of these commands will be verbal as well as a hand signal. Once given the start command you can start the descent. When the bar reaches your chest, you must allow the bar to come to a complete stop, and once it does the referee will give you the press command, where at that point you can initiate the press and start the ascent. Once you have fully locked out, you must wait until the referee gives you the rack command, where then you can rack the bar and complete the lift.
Commands: For squat, you will be given only two commands, and those will be start and rack. You will unrack the bar, get your feet set and everything positioned. During that time, make sure to remain looking down, which is the natural tendency anyways. Once you are set, look up and at that point the head referee will give you the verbal start command along with the visual hand signal. Your next movement must be starting the lift, no more moving your feet or readjusting the bar. You will then complete a full squat, where the hip crease descends below the top of the knee. Once the squat is completed, the referee will give you the rack command where you can then return the bar to the squat rack, completing the lift.
Commands: For deadlift there is only one command, and that is down. You will start the lift at your own discretion, and once locked out and control is shown in that position, the head referee will give you the down command, completing the lift.