using a positive signal as a negative trigger
Simply put, this is connecting a relay in the reverse way it was intended. Normally a ground switch will trigger positive power supply. This is a positive switch for a negative supply. The ground connection can support 40amps (considering you're using a 40 amp relay). I've used this in a few different ways, but 2 applications are listed below.
Applications:
1) I have a positive on/off signal coming from my ECU but I need it to be a negative on/off signal to trigger a device.
2) My brake pedal switch is a positive on/off but I need it to be a negative on/off input for my brake pedal switch input on my engine management.
Make your own custom relay setup
The bosch style relay has two halves, one pulls on the other with an electromagnet, so the coil (pins 85 and 86) is not connected electrically to the switch (pins 30, 87, and 87a).
Pin 86 needs to be a positive wire (20 or 22 gauge).
Pin 85 needs to be ground (20 or 22 gauge).
Pin 30 connects to pin 87a in a 5 pin relay when the relay is in the off state (N.C.). The 87a pin does not exist in the 4 pin Bosch relay.
Pin 87 is connected to pin 30 when the relay is energized.
Extra relay information
Relay Delay: The electromagnetic coil used in the relay is not instantaneous, but it is fast enough that it may as well be. Depending on the relay manufacturer, it will take between 5 and 15 milliseconds for a relay to energize.
Voltage spikes: Relays are known to produce voltage spikes on pins 85 or 86 when the coil is de-energized (also known as flyback voltage spike). It's a tiny amount of current, but this can damage the device triggering the relay if the device isn't built to withstand a small voltage spike. Buy quality relays, some have diodes in them to prevent the voltage spike from the coil de-energizing.
Wire gauges: Pins 85 and 86 can be connected with 20, 22, or even 24 gauge wire depending on the amp draw of the relay. These pins don't require large wires. Pins 30, 87 and 87a need larger wire that correlates with the amp rating of the relay, generally 12 gauge or larger, but check the amp draw of the device you're connecting to your relay before you wire everything together.