There's another really useful strategy with Transfer Only or Attack Only. If you plan to blockade a territory with X troops, you can hit with X-1 troops to either: A) A territory you are 100% sure you will own by the end of the turn using Attack Only, or B) A territory you are 100% sure your opponent will own at the end of the turn using Transfer Only. Then you can move remaining troops to a different territory. This may be tough to comprehend until you use it in your own game or until you see an example. https://www.warlight.net/MultiPlayer?GameID=8878638
Turn 7 is where Timinator used this move. He originally Transfer Only'd 7 armies to Mexico, planning to move the rest to Colombia. If Piggy had not hit, Timinator would have 8 sitting in Panama, and 23 sitting in Colombia, and Panama would be blockaded. It may be tough to grasp this concept until you use it in your own gameplay, so I suggest experimenting with this and seeing how it works.