TC;DR (Too Confusing; Did Read)

Steem pays interest on your Steem dollars every month at 10% APR; however, the interest doesn't appear in your account until a transaction is made. If you don't make transactions often, the accrued interest will not be used to calculate the next interest amount, hence it will not be compounded.

This service, once you register, automatically transfers 0.001SBD to your account ensuring interest is compounded, maximizing your profit without you having to think about it. It costs 0.002 SBD per month to use the service where 0.001 SBD is the cost of the service.

It's called trustless because you don't need to give the service any keys. By transferring a small amount to the service and providing the given MEMO, the service knows who is registered and can transfer to the given account at the right time to ensure interest is compounded.