Both bigs trail the ball and set screens one behind the other for the same handler. The guard uses the first, then the second, and comes off the pair going downhill. Normally the first screener pops back toward the top and the second rolls all the way to the rim.
Why coaches use it
One screen gives a defense one problem. Two screens in a row, with the screeners going opposite ways afterward, gives it a problem it has to solve twice in a second.
Look at the defender guarding the second screener. If he steps up to the ball, he is leaving a rim run. If he stays back, the handler turns the corner with the first big’s defender still behind the play. Meanwhile the first screener is stepping out behind the arc, and somebody has to close out to him from the paint.
It is also the natural transition action for a team with two bigs of different types. The shooter takes the first screen and pops; the runner takes the second and rolls. Both are doing what they are good at.
How it is guarded
Most teams switch the first screen and play the second one straight. That keeps a body on the ball without asking anyone to hedge twice.
Playing both in drop concedes the pull-up between the two screens, which is exactly the shot a confident guard wants. Trapping either screen is possible but dangerous, because the second roller is arriving at the rim as the trap happens.
The counter to a switching defense is to slip one of the two. The counter to a dropping defense is to shoot.