(the following is my take on what the term “Platform” covers in improvised theater.)
The platform is theoretically and in practise what the scene that is about to be created is built on.
Everything that appears to the audience from the moment the light comes on stage and what happens afterwards that forms the contours of relationships is considered the platform for the coming scene.
How are the improvisers placed in the room, are there signs of what location it takes place in, are there signs of the relationship between the characters on stage, are gestures made or something said in the first moments?
All these actions, however small they may be, construct the platform on which the coming scene is to be built.
And it is now up to the improvisers to let the platform prompt them to the following actions.
A new platform arises when a relationship changes. The change in the relationship may be between persons, a person and an object, the weather may change, or a fire may occur.
Now, this new platform must prompt the improvisers until another new platform emerges.