Not all nymphs rise gradually to the surface or wash along helplessly in a current.. Some nymphs swim very well, even faster than the fish that chase them.
May flies can be broken down into 4 basic categories. Swimmers, clingers, crawlers, and borrowers and you'll find each variety in they're own type of environment.
"Swimmers" such as baetis and isonychia are found in the classic pool-riffle-run stretches.
If you've ever seen trout breach right out of the water during a rise, they're chasing fast swimming caddis.
The soft hackle swing is especially effective during such a hatch.