if one plane is on the ground and the other one is in the air there is a lot of strain on the lines of the one flying and may cause them to break. it could cause the plane to crash and both of those possibilities are much more likely to happen as the time goes on so the sooner the grounded pilot can unhook and untangle the better
Dan is on the right track here. I believe the thong stated in F2d and quickly migrated into AMA, as almost every "safety" rule tends to do. The so-called "logic" that if
anybody has initiated a safety rule, it turns into monkey-see-monkey-do.
While thongs prevent the
rare case of a pilot letting go of the handle, they also prevent the clearing of line tangles. The prolonged line tangles, in fact, result in more broken lines and more flyaways. Unfortunately
broken line flyaways are now much more frequent than
dropped-handle flyaways ever were, hence the latest band aid, the centrifugal shutoff.
In F2d, the intential tangle has become an accepted manouver and is no longer even considered unsporting. The Russians pioneered a trim setup that allows the plane to do medium-size inside loops without losing altitude until opponent in taken to Earth. This is an upgrade from the old lazy eights.