I repeat: Do you grant that a pound of, say, Uranium can be used to generate much, much more energy than, e.g., a pound of conventional solid rocket fuel or LOX/Hydrazine mixture?
A couple of pounds of U-238 can provide enough energy (heat - used to vaporize water to form expanding steam to spin turbines) to power a U.S. supercarrier for a year. By the same token, that same amount of nuclear fuel could be used to generate electricity - which would then be used to 1) vaporize lithium (think: electric arc furnace) and 2) accelerate the ionized (and hence now magnetic) lithium using electromagnets, thus providing momentum.
The expanding plasma (cuz it's now more than just a vapor - the lithium atoms have been stripped of their electrons) still wouldn't be all that powerful (i.e., provide that much "push") - but the electromagnets (think linear particle accelerator) do the rest of the job - accelerating them to near light speed.
Can you imagine how much "umpf" even a tiny trickle of ionized lithium would yield when accelerated to, say, 250,000 km per sec?!
That's thousands of times more acceleration than is possible with conventional chemical rocket propulsion systems. The few pounds of U-238 needed for a crewed Mars mission could fit in a lunchbox! It's the electromagnets that consume most of the onboard power plant's output.
There's your explanation - take it or leave it!
Regards,