Rueda eco is similar to Rueda cruzada in the sense that there are two “groups” dancing in the same circle. Group 1 includes the caller and every second couple, and the others are group 2. Group 1 performs the figures first, and then group 2, like an echo. It might not be a surprise that eco (Spanish) means echo (English).
The main difference to Rueda cruzada is that in Rueda eco both teams dance on the same beat, while in Rueda cruzada the teams dance on 1 and 5.