$HYT - BlackRock High Yield Corp. Debt Tax Free ETF

Greetings,

I came across the high yield corporate debt family of ETFs, specifically BlackRock's $HYT fund. Has anyone traded/invested in these? Few questions:

1. They have almost an 8% dividend yield. What are the risks besides the risk on the borrowing firms defaulting, is it more around junk bonds not being a good investment in rising interest rates environments as we perhaps are now in 2016?

2. I believe these are classified as "tax-free" ETFs, what does that really imply?

3. I've always been confused about the expense ratio / management fee percentages listed on these fund prospectuses. I mean, if the fund is trading like a stock with it's own ticker, and is liquid wherein I see the price more every second, what is the importance of the management fee / expenses? Isn't it like, I buy at say $10, and I sell at $11, I make 10% realized gain, pre-tax?

Overall, what's the catch with investing in these funds?

Thanks much.