|How do "forbidden transitions" become less forbidden as pressure builds?
For an atom to determine that it cannot make a particular transition (that its electron cannot move from one particular orbital to another), it must first "test the water". The atom effectively tries to make particular transition but finds that this transition is not possible. However, if the atom experiences a collision during the test period, the atom may "accidentally" undergo the forbidden transition. It is as though the atom was prevented from canceling the experiment.