Pro Site Free Trial Not Getting Activated

Hi, I am using Pro Site, I have a free level and 3 additional paid levels(Bronze, Silver, Gold), I want to give users a 14 day free trial of Bronze level.

I have done these settings in the site:
Free Level name is Free
Free Trial Level is as Bronze
Free trial is set as 14 days
Pay to Blog is disabled
Signup on Checkout enabled
Allow free signup disabled
Allow multiple blogs enabled.

Here is the problem: When i signup for say Silver plan after checking out trial doesn't get enabled but the monthly subscription is activated and the payment confirmation email is received.

Also I am very confused about what following settings actually do, although there is help text for these setting but that's not sufficient for a first time user of pro site to understand what these settings are meant for:

Pay To Blog: What does this module do? What's is the impact of enabling/disabling this module on Free Trial?
Allow Free Signup: What is this? When should I use this?How it works along with the Free Level? What is its impact on Free Trial?
Signup on Checkout: Are there other option for a user to signup when there is a paid plan involved?

It will be great if someone can help with the above.