In gridiron football, intentional grounding is a violation of the rules where "a passer...throws a forward pass without a realistic chance of completion.
"[1] This typically happens when a quarterback about to be sacked passes the ball toward an area of the field with no eligible receiver.
Without this rule, the quarterback could almost always avoid a sack by intentionally throwing an incomplete pass (which would stop the clock and return the ball to the line of scrimmage, avoiding any loss of yardage); instead, the penalty of intentional grounding effectively continues play as if the defense had succeeded in sacking the quarterback.
In particular, spiking the ball automatically results in a penalty of intentional grounding, regardless of any other factors.
However, the rules explicitly allow the quarterback to spike the ball immediately after receiving it from the center to stop the clock[a] without using a time out.