Should we use "trigger event" investment milestones for additional rounds of capital?

One of our advisors recently suggested we consider using "trigger events" for our second round of capital.

Along the lines of "I'll commit $50K now and if you get to 25K registered users by 6/30, I am committed to provide an additional $150K - if you don't, then I'm not committed."

What are best practices here? What are the risks for the LLC and the investors? What are the advantages to both?

It is clearly best for the entrepreneur to get as much money in the door as possible, to maximize runway. But if you're having trouble closing the amount of money you want, there's no harm in setting up a trenched deal (i.e. triggers).

The advantage for the investor is that they are putting money in at series A prices, but a portion of it is at a lower than series A risk level. It's a better deal for them.

The advantage for the entrepreneur is that unless everything blows up you're pretty much guaranteed to get the money. If you get 24.5K registered users you have some real traction. I'm sure the investor will be thrilled to put in the money at series A pricing. They really have to anyway, they won't walk away from their investment.

If things aren't going well, they can walk away. But it would only happen in practice in extreme cases, craziness like Google coming out with a product that obsoletes yours. (And they'll still walk away even if you hit your target if the business really does stop making sense).