With the removal of the glass ceiling, and the scarcity of lava on the map (only two buckets per team), this move is completely obselete.
This is a defensive strategy intended to stop all enderpearl assaults onto a base and allow the enemy team to not see the status of the other planet. The curtain also forces all effective movement to be focused at the glass cieling.
First devised by gk and employed in the monumental Smogon vs OPLV match the plan is to collect enough dirt to create a wall of lava at the front of the planet and later the sides to present stealth attacks. Piston doors have proven to be effective at the planet level but the main exit tends to be a sky bridge.
The main problem with the lava curtain is that the team requires a good bow guard to watch the other team from the top, and there have been moments where the lava has begun to burn the trees, resulting in only front curtains to be used for maximum effectiveness.