How come on older pinball machines you feel a surge of electricity when you hold flippers up?

Pinball Machines utilize electrical solenoids for the flippers.  When it came to the Electro-Mechanical era (everything prior to 1976), the flipper buttons physically completed raw electrical circuit.  This current could be felt in the buttons that's energizing the solenoids.  In such tables, you're likely able to feel the gears & the interaction of components since all there's no CPU to hold the state of the board electronically (it must have a rudimentary mechanical processor to handle this).

In the Solid State era (1976 to the present), the flipper buttons complete a relay connection with the table's CPU.  This disconnects the flippers from the player's direct control (going with a "fly by wire" system) & vastly reduces current required to for the flipper buttons.

Never felt that

but then

I've never used a pinball machine that was older than the 60s

(as far as I know).

The older machines were electro-mechanical.

The new machines are digital, (computer)  devices.

You would probably get a "kick" out of the old, strength machines.