Using bar speed, measured by a device, to set the day's load and to decide when a set should stop.
Every lifter has a fairly stable speed at which their 1RM moves, around 0.3 m/s for a squat and 0.15 m/s for a bench press. Because speed and load track each other closely, measuring the speed of a warm-up single tells you what today's 1RM is, before the client has done a hard set.
The second use is velocity loss as a stop signal. Ending a set when bar speed has dropped 20 percent from the fastest rep produces similar strength gains to grinding to failure with far less fatigue. On a set of squats at 80 percent that usually means stopping around rep 5 or 6 rather than rep 9.
It needs hardware, a linear position transducer or a decent camera-based app, and it is worth it mainly for athletes and strength-focused clients. RPE gets most of the way there for free.