Time under tension is a well evidenced method in which increasing the duration of each repetition or slowing down the tempo of each repetition increases the tension or strain on the muscle. This forces the muscle to work harder overall
This method can be used in a variety of ways, which makes it all the more intriguing. For instance, you can slow down the eccentric and concentric contraction of a lift to produce a new physiological stimulus which you may not have encountered previously.
Take a kneeling push-up for example, rather than keeping the same pace as we move up and down, we can slow down the lowering and pushing movements of the exercise by counting 3 seconds for each phase.
Whereas typically it would take us 3 seconds overall to complete 1 repetition of a push-up (1 second up, 1 second pause, 1 second down), it now takes us at least 6 seconds to complete 1 repetition, greatly increasing the time under tension through varying the tempo at which the exercise is performed.