A little more techie than keithl -
The aircraft end of the DME sends out a signal, which a short time later arrives at the ground transponder. The ground transponder waits 50 microseconds (I think), then replies. The aircraft gets the reply a short time later.
The range calculation is as follows: measure the time duration from initial transmission to final reception back at the aicraft, subtract 50 microseconds, then divide the answer by two. This is converted to distance like keithl says.
If you get the ground station to wait only 10 microseconds before sending a reply, the aircraft gets the signal 40 microseconds sooner, and thinks that it is closer to the ground station. 6000 meters closer actually!
So you can put the DME ground station abeam the middle of the runway, and reduce the inbuilt time delay so the aircraft thinks the ground station is actually at the near end of the runway.
HTH,
O8