said...
Mouse enters, moves the card, second bucket slides down sealing both exit
holes....I've drawn a diagram because I've gone through this a dozen times
before on usenet and a lot of people struggle to understand:

http://tinypic.com/view/?pic=161dwu9

I'm glad you drew the diagram because I was indeed struggling to
understand - although He Who Sits Opposite caught on immediately. Don't
you just love a smartraes.

Why two holes rather than just the one?
--
To reply see 'from' in headers; lose the domain, and insert dots and @
where common sense dictates.