One of my favourite weapons always seems to catch guys behind the guard when whipped in from a 1:00 mark on a clock face. i throw mine almost like throwing a ball in regards to looseness right up until i contact. im also not throwing while rotating around my spine, i try to think of it as touching my right shoulder to my left knee. More of a diagonal twist if you know what i mean. Most important IMHO is not to let your hand leave your face before the start of rotation. couple videos i like
Notice chuck mentions the angle of the punch right away
http://fightland.vice.com/fight-school/the-overhand-left-with-john-dodson
Having trouble finding a working video for this one but in it coach wink says that keeping your hand to your head until right before throwing is super important (start twisting with hand by face then as you dip lover hand flies from there)