In short
A controlled hard brake on the examiner's command, used on about one in three UK practical tests.
The emergency stop is a controlled, firm brake performed when the examiner raises a hand and says 'stop'. The driver brakes with both feet (clutch in only at the very end) and stops in the shortest controlled distance.
About one in three UK practical tests includes an emergency stop; the examiner picks at random. After the stop, the examiner says 'drive on' — full all-round observations are required before moving off.
See also