There are a couple of things you need to be aware of for this article to have any use for you. Firstly you need to be a victim of the clamp', and secondly, you have to be aware of it.
To go off on an immediate tangent for a minute, I had the habit of raising my right shoulder while picking anything fast or tricky. It was an unconscious muscle-tensing response. I only became aware of it because I often wore a pair of headphones that developed a fault that caused them to crackle whenever my shoulder lifted up and came in contact with the wire on that side. If this hadn't have happened, I might never have become conscious of this unnecessary muscle tension, so you could be applying the clamp and not know it.
The clamp is something that affects the fretting hand. For a long time I just assumed it was to do with a lack of fitness, agility, fluency etc. It'll go away with practice I thought. Unfortunately this isn't the case. No amount of grinding away with the clamp on will turn it off and free the fretting hand.
The clamp makes its presence felt in the form of stiffness, aches, the hand locking up, and almost immediate weariness. It's just impossible to play fluently with the clamp on. The clamp is the result of unnecessary tension in the fingers/hand/arm, and the application of far more pressure than is required to fret a string. I'm not sure of the causes, but I think it has to do with trying too hard, getting over-excited, and unconsciously responding to aggressive-sounding music by employing an overly aggressive attacking style of play, so instead of caressing and stroking the guitar, we try to strangle and batter it.
SOLUTIONS; STACATTO SLIDES
Sliding and staccato are two things we can do that immediately release the clamp.
While sliding we instinctively apply just enough pressure to sound the note, but not so much that friction interferes with the slide; we apply the correct amount of necessary pressure, so sliding can be a useful gauge.
The name clamp' obviously refers to the fact that we lock down with the fretting hand in a strangulation type way. This is of course impossible to do while playing staccato, which involves the fingers 'pecking' at the fret board.
It's impossible to peck, and difficult to slide, with the clamp turned on. It's for this reason that playing through some scales, licks etc, using all slides and staccatos gives you a great demonstration of how little pressure should be applied when fretting notes. If the fret board is a surface, and our fingers were feet, we need to tap dance and glide/skate, and not stamp and dig our heels in with lead boots.
One particular area that can lead to clamping has to do with note duration. For example, say we were to play a whole bar of A notes at the 5th fret of the low E string. Each one of the A notes has the duration of an eighth, so we play eight of them one after another. The temptation is to just clamp down the first finger on the 5th fret, and let the picking hand play the eighths, 1 & 2 & 3 & 4 &.
Not only does this one-sided approach to rhythm lead to clamping and cramping, it also means that the picking hand is trying to be rhythmic while the fretting hand just hangs their like a piece of meat. It's like dancing with a corpse. Both sides need to be rhythmically active, so play the eighths with the fretting hand too.
Of course there's a limit to how fast the fingers can fret, release, and fret again. I'm not sure what the limit is in terms of tempo, but it's something that improves with practice, and it's something you need to do as much as is physically possible. It will do wonders for the definition and snappiness of your rhythms, and of course the clamp won't be able to take possession of your hand.
Hope this helps, and apologies for the tasteless analogy.