To get the jackhammer effect when the Yar attempts to eat the shields, I only needed this code :
// code in GML, is executed on collision with shield piece x = x - bugShipSpeed * 3; y = yprevious;
yprevious is a built-in global variable that contains the y location of the game object in the previous loop.
This does exactly what it does in the game. Even forces the yar off the sides of the shield and forces it through it when approached from behind.