I've got an ICS M4 with a forward assist to release the spring tension (prior to storing the gun). However, I'm not sure that the ICS gun's user manual provided any clear instructions on its use.
Someone pointed me to some instructions provided by ICS
http://www.icsbb.com/edm/m4/spring-e.html
Essentially, it states that forward assist should only be used after the gun has been fired in semi-auto mode. If used after the gun has been fired in the full-auto mode, there is a possibility of breaking the piston teeth.