Thorowgood Moveable Block dressage saddles have an optional 'point strap' which can be used to improve saddle stability.
Always start by girthing in the neutral position (2 & 3) with the moveable strap positioned behind the point
If the saddle slips forward, remove the Velcro thigh block, move the billet (girth strap) onto the point (1), passing it through the keeper at the bottom. Reinstate the Velcro thigh block back into position.
DO NOT pass the strap under the block and through the keeper when the billet (girth strap) is behind the point (2). This will cause saddle instability.