sort of OT: repairing drills

Gents,
Trust you all had a good break and had plenty of time to 'play'!
I did the usual round of DIY and in the process managed to half-kill my
drill.
It's an old kango-wolf hammer drill with s two speed gearbox. Now all
is well apart from the fact that it doesn't stay in gear anymore. I've
had it apart and peared into the gearbox, expecting to have half a
spring shoot out at me, but no. There doesn't appear to be anything
missing from the gearbox at all, nothing broken and stuck in the grease
either. My question is this: How does the sliding gear selector stay
where the switch puts it? Does it do it with just the pressure of the
grease inside the box or am I missing something?
I've tried looking on the net, but not found anything yet, and I just
know someone here will know the answer.
Cheers
Andy G