Last summer I had two Italian fisherman who wanted to fish big running trout. I took them to spots which I consider to be nowadays one of the best pool in here. When they booked me, they gave idea that they are professional fisherman which they were but there is also downside. Sometimes those people are not so good to take tips and advices. I picked out some flies (streamers) and we went fishing. I took one of them and told him direction of cast and how to fish, I told him to have speed in fly etc. He casted and right after fly landed he mended. I told him to just take slack line in and let the fly go fast even little bit sweeping. Answer was yes and next cast, same thing fly landed, mending etc. I asked him, if I can show what I mean. I got ”the look” but he gave rod to me. I showed and he said yes now I understand. New cast fly landed, mend etc.
So I gave up with him and went to see how the other man is doing. I went thru same tips with him. He said yes and really start to do like I told. After few cast he had bite. And we all got little bit excited. I checked downstream and saw that other guy started to fish with same technique, no mending anymore but he already missed the spot where not to mend. I left upstream fisherman alone because he got it right and after while he got nice over 40 cm grayling. Of course we were after running trout but those kind of graylings are kind of normal and nice sidekicks. After first runs we climb on cliff and had break with coffee. From there we had nice view to river and we could spot where trouts could be and how river is running. Also we had time to talk about mending when and how. Next day we met on fishing gear shop and I pointed some flies they need to have. One week later when I was in Sweden they whatsapp me with photo, you find it down from here.
For my experience we often mend line even there is no reason. It is more like reflex to mend and slow down fly than there is meaning about mending. It is probably coming because almost all books and articles are saying that after cast mend upstream to fix line etc. You can have same result to just take in slack line (if you are not able to cast that line goes straight) and then you get your streamer swimming not drifting with stream. There needs to be reason for mending and reason comes from reading water and how you want that fly goes. Sometimes you need to mend downstream to give more speed or if current makes eddy or countercurrent. This you will learn by doing and reading water.
Beginners often make mistake and their movements are too fast and powerful. You need to keep your tip of flyline anchored on water when mending (it is same with casts which start from water, there are good videos in https://www.sexyloops.com/flycast/ . If you get your flyline out of water when mending you are also moving your fly. Often it means that you have been able to get fly in good depth and when you mend you lift fly in different level and that shouldn’t happen. I have seen sometimes that during mending fly moves and fish misses the fly. So practise your mending and most of all make purpose when you mend. So your mending needs to be smooth and sensitive, (like how to caress a woman's breasts)
Have a nice weekend