What is a BISS?

Base idle set screw.  This is used to set the basic idle point for your car, around which the ECU will attempt to control the engine speed.  Failure to have the BISS set properly can lead to all kinds of weird idle problems.

Excerpt from: http://www.dsmtuners.com/threads/biss-screw-adjustment.425941/

Adjusting the BISS

1G (since some 1G questions popped up):

To adjust the BISS properly, you need to lock the timing, ground the diagnostic port, and then adjust the BISS until your RPMs are around 750-800rpm (or whatever you have it set to via Link, chip, etc.). If you don't ground the diagnostic port, the ECU will see the BISS adjustment as a change in idle airflow and will attempt to use the ISC to compensate for it. Keep screwing the BISS, the ECU keeps adjusting the ISC with no apparent change in idle rpm...until it runs out of adjustment range.

The other alternative for ECMLink users (as Brian suggested) is to leave it ungrounded and adjust the BISS while watching ISCPosition, until it reads between 30 and 37 or so.

1G BISS Adjustment

For a 2G:

The concept is similar, but I'm not sure about how the OBD-II handles locking the ISC out of the loop. According to the VFAQ, it requires a scan tool, but I've never played with a 2G so I'll leave it at that. The ECMLink method should work the same on both though.

2G BISS Adjustment

Those who need one will find it listed as Mitsubishi part #MD614948, available from any Mitsubishi dealer.