The processors execute the same program in slightly different time steps.
The lead processor runs slightly ahead of the others, especially when it is broadcasting while the others wait for the broadcasted value.
When the program execution accesses an operand that is not owned by the lead processor, a lead change occurs.
All processors stall until the new lead processor catches up and broadcasts its operands.
The capability that each processor may run ahead on computation that involves operands owned by the processor is called datathreading.