Komatsu PC88MR-10 Shop Manual page 183

When boom is in NEUTRAL
If the control lever is returned to NEUTRAL when the boom is raised, the holding pressure on the bottom
side of the boom cylinder is closed by poppet (2).
The pressurized oil flowing in through orifices (a1) and (a2) is closed by pilot spool (3).
The boom is held.
PC88MR-10
10 Structure and function
Control valve
10-105