Rels (бурильные установки)

[ha-ha]

Люди, сведующие в бурильных установках, please help.
Непонятно одно маленькое, но очень неприятное словечко - rels - что это?
Контекст следующий:
It would be very important to have proper training prior to becoming a driller, and working the levers on this drill.

 Some most important things I have learned during my training.
 When using the sonic drill always have (RELS) locked into the top drive. The RELS interlock for proper grip of the drill stem into sonic mode.
 (This can cause extensive damage if RELS are not locked).
 When retrieving drill stem from drill hole always give your top drive room to extend in a vertical motion up. There are rubber pads for the top to rest on. There should always be limited space between rubber rest pads, top and bottom.