oicur 12,
What bekol says is correct.Also from a radar controller viewpoint,if you have 2 aircraft on minimum radar separation,you need to be sure that they will not do anything that will errode that separation.Lock them on headings is 100% safe.
Leaving aircraft on their own navigation,in this situation is an absloute no no when radar training.People have failed validation boards,because not locking them on headings is seen as fail dangerous.Applies to all airports,not just HK.