Tach = tachometer. For model airplanes, its an optical device that when held behind the prop, detects the propellor passing in front of it. They can be had for $30-40, can be set for 2, 3, or 4 bladed props, and indicate how fast the engine is turning. I find it an essential element for fine tuning, since it is much more sensitive to small RPM changes than my ear. I use the [link=http://h1069795.hobbyshopnow.com/products/description.asp?prod=HAN156]Hanger9 ([/link]$35 from Hobbytown) but there are less expensive versions.

Brad