As long as anglers have pursued Steelhead mending the line has been part of the process of presentation. Now one could dig up some encyclopedia reference to the term 'mend' and although it won't say anything about fishing, It will describe the need 'to fix or repair' something.
So this begs the question, 'how does one tell if a cast requires a fix?' Some anglers don't even wait for the need to appear before they start mending line like they are skipping rope, and it's function or effect on the swing varies from beneficial (the first one) to 'hey, what are you doing?'. (the next 14!)
The obvious key to it all is function. What does it do? Most of the time, the mend allows the fly to settle or sink, and even though a sink-tip may be involved to ultimately get the fly down to where it's intended, a proper mend on a weighted fly gets the fly down even faster, or, as in winter Steelheading, as fast as possible. If the fish are hiding under logs on the far bank, this is critical. However, the poor (too hard) mend pulls the fly away from the lie to be effective, and unless you are up a tree or bank watching, this can be oblivious to you. Too many anglers are on the auto-pilot mend program, mending without even knowing why or what for.
I spend a lot of time up trees, since spotting fish and structure they use can be critical to catching them, especially winter-fish. It gives me a unique perspective of water currents and specifically fly speed, the número uno of a swung fly presentation. Too much mending and the fly is constantly catching bottom, not enough, and it's not down in the slot fast enough. In between is the happy place.
The best mend is a SOFT mend, or, as I call it, a 'soft 7'. In other words, cast, let the fly briefly settle and anchor the fly and leader, then softly lift the line up and then left or right depending on the swing direction. Throwing a strong mend too quickly pulls the fly an average of two feet towards you, and out of the potential sweet spot. Another key is having some slack running line in your hand, as you can drop some into the mend to further allow the fly to sink quickly and drag free, but without mending again unnecessarily, then the tension of the current does its thing and the fly begins to swing.
Over mending at this point rarely seems to help unless bouncing bottom is your goal, the relationship of the proper sink-tip, fly and leader obtains the depth of the swing desired, If you want to constantly bounce bottom perhaps a floating line is more you're speed?
Now just to be clear that's called 'nymphing' last I heard and homie don't do dat!