When playing No Limit Texas Hold'Em poker one of the biggest weapons in your arsenal is moving all in. At any point in any hand you can declare "all in" and shove your entire stack of chips to the center of the table, putting an incredible amount of pressure on your opponents. The all in move makes No Limit possibly the most dramatic form of poker, which is one reason why it has such an incredible draw. Most poker advices on the topic will tell you to move all in when you have the best hand. While this tactic certainly has its advantages, there are sometimes reasons to not move all in even when you have the best hand.
In a game like Hold'Em, the best hand at any given time can get out drawn, except for in a few cases. Moving all in can help you take down a pot before you get out drawn, however there are times when you will get called either way. If you are reasonably sure that you'll win the hand you can move mostly all in and have about the same impact as if you moved all the way in. This way you keep chips in reserve in case the cards don't fall your way. If you move mostly in and your opponent just calls you may lose out on some of his chips if you win, but if you lose then you are still in the game.
Moving mostly in instead of all in is a bit of a conservative approach, but one that still has plenty of aggressive elements. If a later player pushes you all in then you can certainly call if you believe you have the best hand. So experiment with a few "mostly in" moves in your next no limit game and see how you do.