A superachromatic half-wave plate is used to rotate the ionising polarisation-shaped laser pulse sequence. This also rotates the photoelectron wave packet. For different orientations of the electron distribution, their projection on a phosphor screen is detected and finally the 3D object is reconstructed. In the animation, actually measured projections and the tomographically reconstructed photoelectron wave packet are shown in the multiphoton ionisation of potassium atoms. If you want to understand how to design specific photoelectron wave packets with tailored light fields, have a look here: Creation of electron vortices