He said jumping, not crossing.

Jumping or leaping is a form of locomotion or movement in which an organism or non-living (e.g., robotic) mechanical system propels itself through the air along a ballistic trajectory. Jumping can be distinguished from running, galloping, and other gaits where the entire body is temporarily airborne by the relatively long duration of the aerial phase and high angle of initial launch.

Some animals, such as the kangaroo, employ jumping (commonly called hopping in this instance) as their primary form of locomotion, while others, such as frogs, use it only as a means to escape predators. Jumping is also a key feature of various activities and sports, including the long jump, high jump, and show jumping.

In quantum field theory, a branch of theoretical physics, crossing is the property of scattering amplitudes that allows antiparticles to be interpreted as particles going backwards in time.

Crossing states that the same formula that determines the S-matrix elements and scattering amplitudes for particle A to scatter with X and produce particle B and Y will also give the scattering amplitude for \scriptstyle \mathrm{A}+\bar{\mathrm{B}}+\mathrm{X} to go into Y, or for \scriptstyle \bar{\mathrm{B}} to scatter with \scriptstyle \mathrm{X} to produce \scriptstyle \mathrm{Y}+\bar{\mathrm{A}}. The only difference is that the value of the energy is negative for the antiparticle.

The formal way to state this property is that the antiparticle scattering amplitudes are the analytic continuation of particle scattering amplitudes to negative energies. The interpretation of this statement is that the antiparticle is in every way a particle going backwards in time.

Crossing was already implicit in the work of Feynman, but came to its own in the 1950s and 1960s as part of the analytic S-matrix program.