To my mind, the world rules are not that clear about this question. lll quoted the section under unsportsmanlike conduct, but this is the actual Use of equipment section:
(c) Mechanical Bridges – The player may use up to two mechanical bridges to support the cue
stick during the shot. The configuration of the bridges is up to the player. He may use his own
bridge if it is similar to standard bridges.
"Support the cue" to me is a little vague, in that even if your hand is in between, you are still indirectly supporting the cue. And they clearly state that the "configuration of the bridges is up to the player." That leaves a LOT of leeway, if you ask me. They should just say something like, "must directly support the cue stick; no device or object of any kind may be used to support the bridge hand". Some of the other rules, like CSI are clearer:
OFFICIAL RULES OF CUESPORTS INTERNATIONAL
10. Mechanical Bridges
The primary consideration for determining whether a mechanical bridge is legal is
that it supports the cue, as opposed to elevating the bridge hand above the bed of the
table or significantly enhancing the stability of the hand holding the bridge.
The particular construction of the bridge is not relevant as long as it does no more
than support the cue, and does not damage the table or other equipment. Items that
are not considered disqualifying include, but are not limited to:
- The overall size, or the height of grooves above the bed of the table;
- whether there is a handle, sliding extensions, wheels, or accommodations to
straddle balls;
- the shape of grooves;
- whether the grooves have linings or mechanisms to retain the cue in the groove.
I put in italics the opening sentence -- you can see they make a point of noting the bridge has to support the cue stick, not the hand.