When you rev your engine, you place additional and unnecessary stress on your car and its engine. This is imperative when it's cold outside—revving your engine before it has had time to warm up is especially damaging, as the engine's oil hasn't had sufficient time to circulate and properly lubricate your car.

What does over revving do to your engine?

Engine damage beyond the redline

If an engine goes overspeed, commonly called "over-revving", damage to the piston and valvetrain may occur when a valve stays open longer than usual. Valve float can possibly result in loss of compression, misfire, or a valve and piston colliding with each other.

Is it OK to rev your engine in park?

It's ok to park your engine in neutral. Don't hold it on the rev limiter when it's cold. It can damage the engine if you try not to.