There are three different types of lassoing tools you select them from the tool bar on the left. The lassoing tool is like a cutting tool it cuts out where you draw. The first one on the list can draw any shape that is on you picture and when you have finished you have to finish where you started.
The second one on the list can only draw in straight lines which is good when you have to keep a certain bit straight.
The third one on the list is like a magnet it sticks to the part of the image you are close to which is better if you are not very good at keeping your hand steady and it keeps it neater.