Forearm pump without finger engagement
Traditional wisdom is that forearm pump is a result of weighting your fingertips for a long time, and you train it by doing hangboard repeaters on 20mm or smaller edges.
However, I don't get pumped doing repeaters, and thinking of the times I get pumped on rock, it almost never involves small holds or finger engagement. I have no doubt that a long overhanging sequence in small holds could get you pumped, but that doesn't occur in the type of climbing I do. If there's small holds, it's on slab and/or in short boulder problem sequences separated by rests.
Here are a few instances when I have gotten pumped recently:
I have a project that's a sequence of hand jams, thin hand jams, and finger locks, and finally jugs. There is not a single small hold in the entire thing. But I get pumped out of my mind, even in the hand jam section. Its steep so there's a lot of weight on my hands. In this case my fingers are straight but are actively pressing against the side of the crack.
Recently I was following a sequence where my right hand did: a juggy hand traverse on a 4" deep lip with an incut edge; then a bomber hand jam in a constriction where my wrist simply slotted in and my fingers didn't have to do anything; then a similar upward-facing undercling-handjam with the weight still on my wrist. Got pumped but managed to hold on by alternately unweighting and shaking out each hand.
In any steep ice pitch I will get pumped while holding onto literal jug handles unless I take steps to address that.
When traversing under a roof on hand jams and foot smears, the fingers could be completely relaxed; the geometry means the weight is just on the "meat" of my hands. But still I will get pumped.
What is causing pump in these cases and how do I train it?
In cases 1&3, my fingers are required to press or wrap around something, and in cases 2&4, my fingers are doing literally nothing.
Hangboard repeaters seem very unrelated as they neither get me pumped and are challenging for another reason(hanging onto small holds with first 1-2 sections of finger). I'm sure they're great for training endurance on small holds, but that's not my problem.
Don't tell me it's overgripping or I'm actually just the world's weakest climber... Anyone will get pumped on a long ice pitch or overhanging hand jams. It's not just me.