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.