What I'm doing is starting here, with a straight upstream cast. I am facing
right against the current; the current is coming right at me. If you don't
strip fast enough, that slack builds up under your rod tip and you lose
control. Not only can't you strike, but you can actually develop drag as
that belly develops below you rod tip. When you cast straight upstream, you
just gather the line as it's coming back to you just as fast as the current
is bringing it to you. That way, that fly is moving perfectly with the
current just like a natural object.
If you take this cast and turn it 45 degrees, that's called quartering
upstream. We have an advantage here, in that it just puts the fly and a
little bit of the leader over the fish and doesn't put all of your fly line
over the fish, so this is quartering upstream. There, usually, you can also
strip line to keep up with it, but the line will start to belly, and then
you may have to do a little mend or something to keep the line in place.
Quartering upstream is a great way to avoid drag, which is an unnatural
movement of the fly, and it can be used with any kind of fly; dry, nymph,
even a streamer. It's probably the most common way of fishing for trout in
a stream, and it works in slow or fast water.