Complex sentence: When applying energy to dihydrogen a molecular electronic transition takes place when one electron in the bonding MO is promoted to the antibonding MO and now there is no longer a net gain in energy.
Split into simpler sentences: When applying energy to dihydrogen, a molecular electronic transition takes place when one electron in the bonding MO is promoted to the antibonding MO. The result is that there is no longer a net gain in energy.