I will give you a situation. If you have a boss who is very irritating. He always blames you with small things like hair style or something. And someday you see the boss is scolded by his boss. than you can meet feeling called 속이 뻥 뚤리는

속이 뻥 뚫리다 is a rather direct description. 속(one's internal system)이 뻥(right away, straightaway) 뚫리다(blockage is removed). 뚫리다 here describes a state that you feel unimpeded and free because there is nothing obstructing your system (either physically or mentally, or both). Since 뻥 indicates suddenness, the phrase describes suddenly feeling good freed from the problem that has been bothering you (It's often used when an indigestion problem goes away). You can imaging a plumber unclogging a blocked drain, in which case we'll say 막힌 하수구가 뚫렸다. 속이 뻥 뚫리다 is similar, except that now it's your body and mind that is unclogged and in good circulation.

I think "cool" is most similar expression :)