In 2024, your business has $1m in revenue and has $2m in expenses. 100% of these expenses are R&D salaries (engineers you hire.)
Your company loses $1m/year. (You brought in $1m and spent $2m.)
Under the old rules, you'd owe no tax because you were unprofitable.
After Sec 174, what the IRS now says is:
You had revenues of $1m. But you only had $400k in expenses (because you now have to spread that $2m in R&D expense over 5 years).
So actually you had a profit of $600k! And you owe tax on that $600k profit (~$120k)
So you now have an additional $120k tax expense, making your business even more cash-flow negative.
.
Amusingly, if you're pre-revenue, none of this matters (you have no income at all, so it doesn't matter what your expenses are.) You get hardest hit by this change when you have some revenue and when you do a fair bit of R&D.