You have it right on the money, except that there is no such thing as a double movement, i.e. it is not a part of the Law.
Law 15 allows the tackled player to immediately "pass, place, push the ball".
[textarea]Law 15.5 THE TACKLED PLAYER
(b) A tackled player must immediately pass the ball or release it. That player must also get up or move away from it at once.
c) A tackled player may release the ball by putting it on the ground in any direction, provided this is done immediately.
(d) A tackled player may release the ball by pushing it along the ground in any direction except forward, provided this is done immediately.[/textarea]
Placing the the ball over the line is legitimate.
What people call a double movement is really "not releasing". If you cannot place, pass or push the ball, you must release. If you propel yourself forward, then you have not met one of the four requirements...... PING!!!