I also use it to finish off. I have sensitive skin so it does sting a bit where maybe I've gone too close. On cuts it stings like hell. But assuming you've had a good burn-free shave, what it's meant to do is act like an astringent on the skin and tighten and tone it all up. You can almost feel your skin tightening after a few minutes.

I have tried witchazel first then alum, and now generally I do alum first then witchazel. The only difference is that the soothing witchazel continues to soothe if you put it on last.

I've also read you can use alum before a shave to tighten the skin. Have yet to try that.

Make sure you wash it off before using other post-shave products because I've found it reacts with most products in a bad way. E.g. D.R. Harris Arlington shave milk goes from nice 'n' gloopy to runny as hell if alum is still on your face.

And don't drop the block. It will shatter and leave you with very sharp edges on the block. I dropped it in the sink, shards came off, too sharp to use on face, and spent 5 minutes grinding down the sharp edges with a pumice stone.

It does also last forever it seems!