ENGINEERING MODE let’s MIUI users to run hardware test of their devices to ensure whether each hardware components are functioning properly or misbehaving
Go to Settings -> About Device, then keep tapping on Kernel Version unless you trigger ENGINEERING MODE window.
Method 2 (May not work on all devices):
Go to Dialer and type *#*#64663#*#*. If it doesn’t trigger ENGINEERING MODE automatically, then press call button.