From Intellivision Wiki
|CP1610 Clock Cycles||Infinity|
The Halt (HLT) instruction halts the CP1610. The CPU asserts its HALT output, and performs no further processing once it executes this instruction. The CPU ignores interrupts as well until the CPU is reset, or unhalted by a negative pulse on STPST. HLT is mainly useful for debugging.
A pulse on the STPST input will restart a halted CPU. On the Intellivision, this is mostly irrelevant as STPST tied to a fixed value. The CPU is socketed, however, so one could modify their Intellivision in order to make this useful, say with a "debugging support" daughtercard.