Diode test mode provides a current to the 2 leads, and inside the adaptor there is a resistor from either collector to base for NPN or a resistor from collector to base for PNP, and 2 sockets labelled as required. Exactly the same as the method used on the 830 clone meters, just with a removable socket. The meter provides a current source to the mA socket using the mA mode sense resistors to read it, and feeds the current into the normally grounded side of the resistors and uses the 10A socket as a convenient way to get the return current, as the common terminal is too far away.
The resistors do not affect use as a K probe socket either, they only are in circuit with a transistor.
Just note that there is absolutely no specification given for transistor test at all, it is just there to fill a switch position, the readings will have very little use in real life other than showing a transistor is either a unit with some gain, is not totally shorted but there is absolutely no difference between a leaky transistor with no gain and a working one with gain. It just says it passes current. Might be usable as a LED test fixture instead if the voltage open circuit is high enough.