The idea is that typically people expect a laptop to remain powered when the cord is unplugged (due to the battery), and so in a situation where the laptop is grabbed the power is cut immediately (and unexpectedly) and the hard-drive is no longer decrypted. As another commenter pointed out, a forensics team could then probably grab whatever they needed from the system memory (presuming they acted fast enough), but as I understand it the problem becomes significantly more complicated.