2003 4.3 V6 starting question

I have a 2003 Base model with the V6. When I start it, I usually just turn the key to "start" very briefly and let it return to the "run" position and the engine fires right up. Occasionally, I do that, and the starter stays engaged for a 1-2 seconds, only until the engine starts. Is it supposed to do that, or only crank while the key is held to the start position? It seems to stop cranking as soon as the engine catches, and has never taken more than 2 seconds to do so.