The squash fold is a pacific type of fold used on two layered triangular flaps, like the ones fond on the preliminary and water bomb bases. It involves lifting the flap in question, separating it’s two layers with your fingers, and then pushing down the top crease of the flap so that the flap is squashed into a flat triangle, with two new flaps on each side.
This fold is fairly hard to visualize and to do accurately if you are not practiced in making them. If you would like a more hands on example, follow the below instructions on doing a squash fold on a [preliminary base.
For this example, you will first need to fold a preliminary base, follow the link for instructions. Select one of the front flaps, and lift it so that it is is at a right angle to the rest of the base, over the center of the base.
Then, insert your fingers between the layers of the flap, while keeping the rest of the base from moving. Then, once the flap is fully open, press down on the end of the flap, where it comes to a point with the other flaps of the base, and squash down the top crease of the flap along the center of the base. While doing the above step, firmly crease the folded edges of the new triangle that is forming. Once you have squash down the flap along the entire top, it should resemble a flattened isosceles triangle, centered at the central crease of the base, with two right triangle flaps on either side.