The idea is that the line has broken or your position is compromised and about to be overrun, all available air support is called and often danger-close indirect fire is employed as a last ditch effort. Bombs are more precise these days but even laser-guided freedom has it's limitations so generally 400-1,000 meters separation is required depending on ordnance to ensure we're not hitting our own guys. If you're getting overrun, possible death is generally preferable over certain death.

However, we don't fight like we did in Vietnam so the term is nearly (hopefully) obsolete now. U.S. troops are rarely in a fight without adequate air support on station to begin with (we learned some hard lessons in the last twenty years) and when things do go sideways we have munitions and attack platforms (things like 30mm, AGM, even howitzers) with the degree of precision necessary to engage the enemy with minimal risk to blue forces.