Note that the trailers, when they walk forward to meet each other, meet with the same hand they were using to form the box originally, and they come back out to meet one of the original leaders with that same hand. As long as they are careful to walk straight ahead both at the beginning and the end, without veering or turning, this will all happen automatically.
This means that from a right-handed box, everything will be happening to the right. The leaders will be flipping over to their right, the trailers will step ahead to right hands, and everybody will come back to right hands at the end.
In a Scoot Back from a left-handed box, the leaders will be flipping to their left, the trailers will be doing the trade in the center using their left hands, and everybody will come back to left hands at the end.
It is very common for Scoot Back to be done by everybody in the square at the same time, from a position where there are two boxes of four. For example, in the case of parallel waves, there are two boxes side by side. Each box does the Scoot Back separately. Nobody from one box moves into the other box.
Scoot Back can also be done by a group of four dancers with a mini-wave in the center and the other two dancers facing in towards them. In this case, the center and outside dancers walk forward to meet each other, forming a temporary box using the same hand as the original mini-wave, and everybody turns 1/2. Then they drop hands and walk forward again. The dancers who were originally in the mini-wave will be back in a mini-wave, holding the same hand. The others will be standing individually again, but those dancers will now be facing out, rather than in.