How can you tell?

Heywood Floyd:
[referencing Discovery's schematic diagrams]
Are these the power connections for all the control-based circuits? Walter Curnow:
Most of them, yeah. Heywood Floyd:
What other ones are there? Walter Curnow:
Well, all the environmental controls are fed with this one here... Heywood Floyd:
Yeah yeah yeah, but this is the one that feeds into HAL, right? Walter Curnow:
Right. Heywood Floyd:
Okay... I want you to install this little beauty... right about... there. Just inside the cable trunking. [hands a small device to Curnow] Heywood Floyd:
I want you to put it where no one can find it without a deliberate search. Walter Curnow:
No shit? Heywood Floyd:
No shit. Walter Curnow:
This is pretty sweet... Non-conducting blade, so there won't be any short-circuits when you trigger it... Where's the remote control? Heywood Floyd:
If I trigger it. The control's in my compartment. Little red calculator? You've seen it. You put in nine '9s'. Take the square root, and then hit 'Integer.' In an emergency, even you could do it. Walter Curnow:
What kind of emergency? Heywood Floyd:
Well if I knew that, I wouldn't need this stupid thing now, would I? [holds up the device] Walter Curnow:
Y'know, Chandra'd have kittens if he found out. Heywood Floyd:
Yeah. But he isn't gonna find out, is he. Walter Curnow:
Not from me. They could tear off my fingernails, I won't talk. Heywood Floyd:
Good. Install it tonight when he's asleep. If he ever does sleep. Walter Curnow:
How can you tell?