30 C.F.R. § 75.1726
Performing work from a raised position; safeguards
(a) Men shall not work on or from a piece of mobile equipment in a raised position until it has been blocked in place securely. This does not preclude the use of equipment specifically designed as elevated mobile work platforms.
(b) No work shall be performed under machinery or equipment that has been raised until such machinery or equipment has been securely blocked in position.