Landing fish hand-lining is a totally different mindset than bringing a fish to net on a short line.
I tend to prefer shorter lines that do not require handlining as I feel it is better for the health of the fish. Bring them green to the net and release them as energetic as they were when I landed them.
Sometimes even with a short line, I handline because the fish are so big they cannot fit in the net.
So long line hand line or short line handline, part of the best technique for me is to identify when the fish is ready to land. This requires patience as it will take longer than it takes to shortline a fish to net. The best identifier is that I can easily lead the fish and can tell that I have broken it’s will. The time this takes is individual to the fish and to the water in front of me. A key identifier to this is if they are comfortable being in shallow water near the bank. They can still be pulling, but not frantically trying to flee. There is not a fish that is not completely spooked by shallow water. You dont want to completely toast them either, but if they can be guided into this danger zone then they can probably be easily and quickly handlined. In deeper water if you are wading, I just wait until their desire to run subsides.
Sometimes I start to handline and see the fish is not even close to ready and I just let the line slip through my fingers until all the weight is back on the rod. Identifying a good landing spot is really important. Soft water or a small eddy in hard water are good places to consider. In deeper water I try in the eddy i create. Depending on the size of the fish, I will even walk through a bit of water just to land the fish.
The key to all of this is focus and deliberate relaxed action. Neither rushed nor too casual. When I handline I always consider landing the fish improbable as I fish barbless and the fish has the physical potential/upperhand to get away. Thinking this way it removes any anxiety from the situation and really increases the probability of success. Really gentle tension is all that is needed and the fish rarely puts it together it could simply run directly at me or go airborne.
The general hold is loose in the free hand, so if they do run the line will slip through my fingers. Feed each retrieved length to hold in a couple fingers on the rod hand. If the fish lunges the fingers release the lengths in sequence. Sometimes just letting the line slip through the retrieving hand a couple inches is all that is needed. It depends how green they are. You really need to use a light pressure so the fish does not react. If the fish is still really green, you are better letting the rod tire it than force it to handlining too soon.
Super important tip. While it is fine to lift the head slightly out of the water when engaged on the rod, as the rod can absorb sudden lunges. I try to minimize any lifting of the head of the fish and try to pull at a lower angle. A fish with its head out of water can get agitated easily, causing a lunge or even a jump. Both will often result in a lost fish, but not always. A slow deliberate pull closer to the plane the fish is on is much better.
I hope it helps