Lilmissflamethrower

I was watching a Brittish series called Danger UXB. Its about bomb defusers in WWII England.

Anyway, in one episode, the unit was having an inspection. I believe they called it a "parade."

One of the guys was lighting his boots on fire. I mean he was holding the boots up to a flame.

When the CO came to inspect the group, he took the boots in question off the bed and dropped it into a bucket of water. Then he started yelling at the guy. Their accent is so thick I couldn't tell what he was saying.

My questions:

1. Why would you hold your boots up to a flame?
2. Why would that be bad?
3. Also, while I am asking questions about WWII and English military, what is a sapper?

Thanks! Stacey