I would like to know what 8051 does in ONCE (on chip emulation)

Having checked the reference given by Erik (always a good idea), I see that this should be "ON Circuit Emulation" - not "ON Chip Emulation"

So this is not a feature within your 8051 providing built-in debug/emulation facilities; rather it is a means to disable your 8051 while it remains fitted on your target board, so that an emulator can be plug on top of it - taking over its functions.

The Triscend E5 and similar are entirely different; they do have built-in debug facilites within the chip - there is no need for any additional plug-on hardware (apart from the JTAG connecion, of course)

My mistake.