Action name: ToggleTraceInstructionsThis command starts instruction tracing. You can then use all the debugger commands as usual: the debugger will save all the modified register values for each instruction.
When you click on an instruction trace event in the 'Tracing' window, IDA displays the corresponding register values preceding the execution of this instruction. In the 'Result' column of the Tracing window, you can also see which registers were modified by this instruction. By using this information, you can for example quickly determine which instruction modified a specific register, or you can even backtrace the execution flow. Note that the IP register is never printed in the 'Result' column, although it is usually modified by almost any instruction (except perhaps some prefixed instructions like REP MOVSB, ...).
Internally, the debugger runs the current thread step by step to properly obtain all the required register values. This explains why instruction tracing is slower than a normal execution.
See also Tracing window Trace management Clear trace Function tracing Basic block tracing Add write trace Add read/write trace Add execution trace Tracing options Tracing submenu.