Not quite. The way that it works is that the 375k is invested now, but at terms that are determined in the next equity round. If the next round values the company at 10 million, then the 375k would be 3.75% of the company.
That's... almost unbelievably founder-favored, yeah? Neat.
The company gets the money now.
The more they grow, the less YC gets for the 375k. But the more they grow, the higher the value of 7% is going to be. And also, the more they grow, the more they are likely to grow in the future. So the 375k share is also more likely to keep growing.
So in a nutshell: the 375k is incentive for the company to grow, which is also in the interests of YC, since they have 7% (+ x%) and in general getting startups to grow is the whole point of YC.
You're just providing an alternate scenario that isn't as favorable. And since the initial $125k implicitly has a $2m valuation attached to it, if you raise again at $3.75m, then that's probably not ideal.
So a sensible approach would be to view this as providing an implicit minimum value to target for your next round, i.e., >$5m (7.5%).
As per https://www.ycombinator.com/deal “The $125k safe and the MFN safe will each convert into preferred shares when your company raises money by selling preferred shares in a priced equity round, which we refer to below as the “Safe Conversion Financing” (this will typically be your “Series A” or “Series Seed” financing, whichever happens first).”
Edit: Sorry, I am absolutely wrong here. I completely misunderstood what nirmel was saying.