and how can you even tell the pilots did “grasp and hold” during an investigation? Is it determined based off what they say?
The FDR would show exactly what the control inputs were so they would know from that.
It's important to understand that pitch trim effectively controls the airspeed the airplane will seek naturally without any control input (FAR 25 pitch stability requirements specifically require airplanes to effectively seek a trimmed speed within certain parameters when the airplane is sped up or slowed from trim speed and left on its own).
If trim speed is higher than the actual speed, the airplane will pitch down (trying to accelerate to its trim speed) and vise versa if trim speed is lower than actual. If you are trimmed at 180kt, the airplane will do whatever it has to do to maintain 180kt by going up or down hill (set aside thrust for now). If you move the trim nose down, you are effectively telling the airplane to seek a higher speed, and it will oblige by pitching down until it's at its new trim speed and then stop changing in pitch.
If you are trimmed out at 180 kt and the trim "runs away" in the nose down direction, what's happening is your trim speed is increasing, and if you disable the trim to stop the runaway, you are now stuck with the new trim speed set by the position of the now disabled trim system.
If it runs away nose down and you kill it a couple seconds later, you had an airplane that wanted to seek 180kt "hands off" now wanting to seek some higher speed, say 250kt. To keep 180kt, you have to apply a continuous elevator input, basically counteracting the stabilizer. You will have a stab that is set more leading edge up than it was, while you hold the elevator more trailing edge up (you have to hold it displaced from neutral relative to the stab surface) to keep flying at 180 kt, and if you let go, the airplane will do what it wants to do, which is seek 250kt (in this example), and it will pitch over and accelerate.
So when you have a Nose Down stab runaway and you disabled the system, you are now stuck having to hold up elevator input continuously to fly any slower than the speed set by the stab's new position. This can be up to 50lbs depending on how far apart the trim speed and the speed you want to fly at is (on a jet with hydraulic controls there is no aerodynamic feedback and you are just pulling against an artificial feel system, typically springs of some kind).
So there you are, "grasping and holding" the elevator to keep the airplane from pitching over to seek the new much higher trim speed that was set by the stab moving unexpectedly nose down until you killed its motion. It might be, say, 30 lbs. Your arms get pretty tired and you have to take turns.
Therein lies the temptation to maybe try to cheat the stab back to a lower trim speed by reengaging it, even though it violates your procedures. In the case of the 737, you have manual trim capability, but it gets really difficult to move due to the load on the screw jack if actual speed and trim speed are far apart so it may not really be available (you actually have to let the airplane go and pitch over and maybe even help it along with elevator to offload the screw jack so you can move it - not feasible when close to the ground).
Stab trim runaway scenarios are done in the sim during training and recurrent, but that's normally a simple runaway, clacker goes off, disconnect, and deal with the situation from there. In the MAX case it wasn't a typically simulator training runaway situation; the MCAS system was doing its weird timed trim pulse thing when it shouldn't have been, and the crew didn't even know the system existed, so they're doing their stuff with a strong "what the hell is going on?" factor added, with an airplane originally trimmed for climb speed now wanting to go a lot faster unless elevator is held, sowing the seeds of confusion and increasing the risk of brain-lock (I've experienced brain-lock in the simulator), which seems to be what resulted in these cases.