Elastic effect occurs as a result of the relative difference in the skills. For example, consider a center with sensational IS, ID and RB, and say his SB is only respectable. Since SB is waaaaaaay below other skills, when training SB there is an elastic effect and it will be trained at a quicker rate.
The opposite is also true. If a player has respectable IS, ID and RB, and his SB was sensational, training SB will be slower because SB is waaaaaay higher than other skills.
Cross training is completely different from elastic effect (at least that's how I see it). Basically some fraction of the training goes to training any random skills. So you may be training stamina but a player could pop in JS as a result of that fraction of training being used to train JS.