When you rotate your bowling arm while your non-bowling arm is stretched out at 90 degrees, the non-bowling arm(provided it's firm which I believe it is since it's stretched out at 90) will naturally rotate downwards(clockwise towards the body) thus creating forward momentum.
Bonus: If the arm is raised higher than 90 and thrusts downward(clockwise towards your body), the greater the force. Watch Johnson, Akhtar and Thomson as examples.
I haven't seen your previous action therefore it's difficult for me to explain the difference thoroughly. But what I can assume here is that your previous action didn't allow your bowling arm to rotate as fast and as freely as your current action allows.
Also by keeping your arm firm at 90, naturally thrusting downwards as you bowl, though the range of motion isn't as great, it's generating more force than your previous action due to the thrust a firm arm creates. You might have used your front arm in your previous action but more likely for show than to generate any amount of force.
You mentioned you're bowling yorkers a lot easier now. The reason is because your action is completing earlier.
The earlier/higher the ball is released the higher the trajectory therefore the ball is pitched further up.
Your current action completes earlier because your front arm completes its motion faster as there is less distance to travel, as a result the body stays more upright and there is less bending of the back compared to the previous action and the ball is released earlier at a higher point.
Remember to eat your beef. 
