Cross-band repeater working process.

Ham radio VHF/UHF crossband repeater operation is a common use for amateur operators.and it is an effective way to extend the range of your handheld radio. 

Normally Cross-band repeat function will be taken by dual-band/dual monitor mobile radio and a dual-band or UHF/VHF capable handheld radio. Cross-band repeat only works with a two simplex frequencies (one UHF & one VHF) or a UHF simplex and a VHF repeater pair frequency.  You will use the UHF channel on your handheld to communicate to the dual-band mobile. The cross band repeater capable radio then takes that signal and simultaneously transmits it on a VHF frequency (either VHF simplex or repeater). When the VHF station or repeater makes a transmission, the cross band repeater function radio receives and retransmits the signal on the UHF simplex frequency. This is the cross-band repeat process.   

Another important point is The receive sides of both VHF and UHF on the cross-band should have CTCSS (PL) tone encode set to minimize the chance of false triggering by other signals.