Pipe light (figure 1) describes the condition of the well which well head pressure and buoyancy acting upward against a cross sectional area of pipe is greater than weight of the string. If the light pipe condition is happened and the string is not controlled by the snubbing unit, the string will be pushed out of the well.
Figure 1 – Diagram for Pipe Light Condition
You may see these videos showing the pipe light condition. These videos below clearly demonstrate when the pipe is in “Pipe Light” condition. You can see that the tubing string is hydraulically pushed out of the hole by wellbore pressure.