Criss-cross in freestyle slalom: legs cross alternately between the cones. Placement in the World Skate and SkateIA systems, build-up via fish, mistakes and safety.
The honest answer up front: in the criss-cross you cross your legs alternately over each other while skating between the cones. The World Skate trick system (Appendix A of the freestyle rules) lists "Cross / Snake / Fish Series" at the lowest level of the matrix (class E, level 1) [2]. The grading program of USA Roller Sports and SkateIA names "Crisscross" and "Backward Crisscross" in Grade I of eight levels (as of 2017) [1]. In plain words: it is the first crossing trick, no master move. It is still a good test of whether your balance is enough for crossings.
Why does it become a test? When crossing, your standing leg carries a lot of weight and balance is decisive. In a survey of 313 recreational skaters, 32.9% named a spontaneous loss of balance as the cause of involuntary falls [4]. The DILV curriculum knows the pre-exercise: in the egg walk ("lemon riding") the variation "same exercise, but now slightly crossing the skates" follows [3].
How hard is it? You understand the principle in one session. Until the crossing at speed between four cones runs cleanly, it takes a few weeks of training, in experience terms. There is no study on the technique. Anything without [n] is practice standard.
Place four to six cones at 80 cm spacing in a line, as in the World Skate system for rating [2]. A tight, even spacing forces clean technique. Milestone: you skate fish through all cones.
Practice the crossing first standing and walking without wheels: the right leg crosses over the left, then the other way round. Then repeat it slowly on wheels without cones. That prepares you in a way similar to the slight crossing in the egg walk [3]. Milestone: 10 crossings in a row without a skate rolling away.
First skate through two cones: before the cone one leg crosses over the other, after the cone it crosses back. Stay low, upper body calm. Anchor: you draw an X between the cones.
Extend to four cones and find an even rhythm. If a leg catches, take speed off. Milestone: you manage 4 cones without putting a foot down, 5 times in a row.
Switch the starting leg and direction. The USARS/SkateIA program requires "Crisscross" and "Backward Crisscross" for Grade I [1]. Backward comes only when forward crossing sits securely.
Film yourself from the side and from the front and look at the crossing: does the leg cross at the same height, with a similar distance before and after the cone? If so, your rhythm is good. Also do a series in which you skate only 3 cones, but 20 times without putting a foot down. Consistency before length: that is the currency in slalom.
A trick in which you cross your legs alternately over each other between the cones. It belongs to Grade I in the SkateIA system [1].
It is among the entry tricks: World Skate lists "Cross / Snake / Fish Series" in the lowest class of the matrix [2]. For beginners on skates, crossing is still demanding.
The World Skate system rates tricks on cones with 80 cm spacing, at least four cones [2].
Yes, as a prerequisite. Fish and snake are the base on which crossings build.
Eagle in Freestyle Slalom: Feet Turned Out, Cones in Rhythm