On a microscopic level, the corn flour goop consists of small starch particles packed close together. Separating the particles is a thin layer of water that acts like grease – allowing the particles to slide across each other and move around, as long as they move slowly. So, when you slowly push your fingers into the goop, the starch slides out of the way, allowing you to slide in easily. In this situation, the fluid applies viscous drag to the grains gently slowing their motion. However, if you try to smash your fist in quickly, the starch tries to move faster than the water can accommodate and grains come into contact. Now, the much stronger force of static friction acts between the grains – as long as they are being pushed together, there is force preventing them from sliding across each other - and the harder they are pushed together, the stronger the friction force is!
Almost instantly, long columns of starch grains are pushed together – a chain reaction of jammed particles that are held together by the stress you are applying (the force downwards from your hand) and the frictional forces that stop them slipping sideways out from under your hand like they did when you moved slowly. This "jamming" leads to "force chains" through the goop. While the stress is applied these force chains can last essentially forever, because of the static friction. Releasing the stress allows the structures to break down, returning to its fluid-like state.