it is like bench pressing you want your wrists and elbows to be aligned, ie your forarms need to be verticle, not bent backwards.
If your elbows are forward it places more pressure on the elbow joint, your delts are already 30+% contracted, and the begining of the lift is almost all triceps.
When your forearms are verticle, your triceps and delts contract at a similar rate, more muscles working at the same time makes the lift more efficient.
I know when I start my triceps are against the side of my body. I dont know how you could get the bar on your upper chest with your elows forward, it would be more on your delts / fingertips like the final clean position