Here is a quick video on how training can be used with the EMS.
As a full disclosure, you can never replace speed or strength training with your actual event-specific discipline (i.e sprinting fast to run fast), thus the EMS is used to “top up” on your training.
However, when you have to miss a workout due to bad weather or travel delays, at least you can get a training session in.
For training, EMS can provide greater contraction as compared to a normal voluntary contraction, up to 30% higher. (i.e. more muscle fiber recruitment)
In addition, EMS can be used in training to alter the order of muscle recruitment.
Normally, your body would use red fiber (slow twitch) first to do a specified movement, followed by white fiber (fast twitch) when needed.
However, with EMS, all fibers in proximity of the electrode pad regardless of their type are recruited. Therefore a greater number of fast-twitch fibers are activated earlier than they would be with traditional training. This is beneficial for speed, power and strength athletes.
This article is a great summary on the many uses of electric muscle stimulation.