The kind of tip sheets I am familiar with are related to copywriting and e-zines. The theory is - people sign up for your ezine, you send periodic tip sheets to the subscribers through their email. That's in addition to the newsletter that they already receive. It's promoted as relationship building, keeping your name fresh in your subscriber's memory, and keeping them tuned in to your product offering - whatever that product might be.
Some examples:
A gardening ezine - tip sheets on how to overwinter perennial herbs, making a topiary, which herbs are best to start from seed, which plants novice gardeners should never waste time trying to grow from seed, etc.
A bike enthusiast ezine - tip sheets on 'best' new products, how to overcome hitting the wall in altitude biking, how to train for a race, etc.
A copywriting ezine - tips on why headlines are powerful and how to make the most of them, why use bullets and how to use them effectively, how to use bits of personal info in your ezine or newsletter to make subscribers feel they 'know' you, etc.
That's basically what I've seen/heard about tip sheets. Some people swear by them. You can PM me if you want specifics. I'm not free advertising for anybody. (Especially not here.)