Heartsteel is the new Economy Trait in Set 10 and it is quite capable of winning a Lobby if you know how to abuse it. You always receive a cashout after 4 rounds of fielding Heartsteel and you can improve the cashout by killing units and/or losing fights. Running more Heartsteel will give you a multiplier to reach even better cashouts.
The best way to utilize Heartsteel is to play around Spatula. If you have an Emblem by Stage 3-5 you can craft a Heartsteel Emblem together with a Belt and set yourself up for a large Stage 4 Pivot. Push for Level 7 in Stage 4-1 and roll for any Heartsteel Chosen (Aphelios/Sett/Yone). Find Ezreal, give him Items and put the Heartsteel Emblem on Kaisa/Miss Fortune/Mordekaiser.
Since you always receive a cashout after 4 rounds, it's important that you DON'T run Heartsteel in Stage 3. You will end up losing HP with a weak board for a mediocre cashout and in the worst case delay your large cashout from 7 Heartsteel in Stage 4. Instead you want to run your strongest possible board in Stage 2/3 and stay high HP with a win streak. This way you can afford to take losses in Stage 4 and build towards a large cashout.
In late game you want to push towards Level 9 to add Kayn and Yorick. You can sell your Heartsteel Chosen in late game if you completed a Cashout and want to field stronger units to win against the final opponents. With Champion Duplicators you can also aim for Ezreal*3 as a win condition if you are uncontested.