Question regarding service rule

Several guys in my club say that my service is a violation. In double backhand service, my left wrist (non shooting hand) has a slight upward flip and then right hand racquet hits the base of the birdie almost at the same time. They are not complaining over waist, over hand, not hitting base, delay or anything else, they say that my left hand can not flip the bird upward, I can only release it. In that sense, I cannot throw the bird up to the air, wait until it's low enough and hit it either. Mine stroke is a mini version of that, although I hit the bird when it's going upward.

So I check the rules tonight, it doesn't even mention the non-shooting hand, am I not getting the complete rule, or their interpretion is wrong? Thanks for any comment.

The service rule I found is listed below.

11. SERVICE

11.1 In a correct service:

11.1.1 Neither side shall cause undue delay to the delivery
of the serve.

11.1.2 The server and receiver shall stand within diagonally
opposite service courts without touching the boundary lines
of these service courts; some part
of both feet of the server and receiver must remain in
contact with the surface of the court in a stationary
position until the service is delivered (Law
11.4);

11.1.3 The server's racket shall initially hit the base of
the shuttle while the whole of the shuttle is below the
server's waist;

11.1.4 The shaft of the server's racket at the instant of
hitting the shuttle shall be pointing in a downward
direction to such an extent that the whole of
the head of the racket is discernible below the whole of the
server's hand holding the racket;

11.1.5 The movement of the server's racket must continue
forwards after the start of the service (Law 11.2) until the
service is delivered; and

11.1.6 The flight of the shuttle shall be upwards from the
server's racket to pass over the net, so that, if not
intercepted, it falls in the receiver's
service court.

11.2 Once the players have taken their positions, the first
forward movement of the server's racket is the start of the
service.

11.3 The server shall not serve before the receiver is
ready, but the receiver shall be considered to have been
ready if a return of service is attempted.

11.4 The service is delivered when, once started (Law 11.2),
the shuttle is hit by the server's racket or the shuttle
lands on the floor.

11.5 In doubles, the partners may take up any positions
which do not unsight the opposing server or receiver.

 

I mentioned it, didn't emphasize it though. They are not complaining over waist, over hand, "not hitting base", delay or anything else.

"not hitting base" means I always hit the base of shuttle cock first.

All they said was that I shouldn't flip or toss the bird in any extent, all I can do with my non-shooting hand is to let the bird go. And apparently the rules didn't mention anything about non-shooting hand at all. I think they are wrong, any more comment? Thanks for all the reply.