With the case cover removed (two screws), inside the IBM 5100 is a special control switch to enable RUN vs STEP mode. When in STEP mode, operators press the white switch to then execute one instruction. The system is then suspended once the instruction is completed.
In between STEPs, the display is updated and the state of the registers. So, the current state of any register value can be observed while in the STEP mode.
A single instruction is both an Instruction Fetch and Instruction Execute cycle.