What do you use to stop someone from advancing?

A beautiful end to an ugly start, or at least that's how the tale goes. But dive in without water, and I'm a beautiful start to an ugly end. Oddly, I don't really do the action that was named after me.

With a pole up my backside. I feel no pain. That might change If I only had a brain.

A whole-y cone, useful in the alchemist’s tools, Filling flasks, gas cans, a mechanic’s dream.

You push me down. Then you wait. Surprise!