EMS can increase strength by improving muscle efficiency.

That is, by “teaching” your muscle fibers to fire more effectively.

When the brain tells a muscle to contract, many of the muscle’s individual fibers fire, but, in most cases, some are held in reserve to take over when those doing work get tired out.
EMS, on the other hand, causes all of the individual fibers in a muscle to contract at once, which is why studies show that it can activate about 30% more fibers in a given muscle than simply flexing it.