If he stole Superman's speed, why would Superman keep moving? If his kinetic energy was stolen, Superman should be frozen perfectly still in his last position and unable to move; not hurtling into a mountain. If Superman flew into a mountain, he still had kinetic energy.
If he stole Supermans speed, Superman wouldnt be able to move willingly. Thats like you running at your top speed and all of a sudden ankle bindings appear. You'd fall and roll a few feet. Thats the same that would happen to Superman. But he'd roll for thousands of miles. Thats what Flash was saying basically. He could steal speed energy from someone who isnt moving at all. But in Supermans case, because he was moving at such high speeds, that would cause the destruction from Superman's roll.
Superman's legs wouldnt be able to move at all. I wouldnt call it tripping because its actually making Flash faster. So its still technically stealing Supe's speed. But Superman's body motions would appear as if he was tripped. But in reality, hes just stopped in mid motion thus making him fall at 2,000 miles per second.
Flash v.3 #136-8 -- Flash evenly races an energy being who moves at or beyond lightspeed, then, boosted by the collective speed of humanity, outruns "instant transmission' that is specifically referred to as being beyond lightspeed.
Thats one.
Flash v.3 #132 -- Flash, boosted by Johnny Quick's Speed Force equation, moves so fast that time stands still for him; this is, in Einsteinian physics, the same as lightspeed motion. From this issue on, lightspeed travel becomes routine for him.
Theres another.
Thats 2 instances where he actualy steals or borrows speed. Superman would have been another, but the destruction would have been too high. Is that good enough?
"BOOSTED" as in taking someones speed, kinetic energy or momentum.