Caps over Sherline spindle.. doesn't work...

Well, tried it and.. well, it doesn't work.
No smoke, but rather scary: I started with the motor down on low speed,
turned it on.. it ran really slow for a second, then with a THUD (and
lights dimming) the motor raced up to maximum speed. The motor seems to
lock at max speed... of course, I didn't wait around like that. I tried
it again with the same behavior, then removed the caps and it's still
working.. which is good.

So why didn't this work then? Does the Sherline speed controller do some
trickery with capacitance/resistance to compensate for heavy-cuts and
thus deliver more power when the motor is under load? If so, what can I
still get away with on this motor and caps? Or is it a dead duck?

Thanks guys!
*confused*
Abby