Why is "brake" suddenly occurs when shifting between the 1st and the 2nd gear of the wheel loaders?

If the control lever assembly (shift lever, etc.) of the transmission control valve is loose or improperly adjusted, the pressure oil will enter the first and second gears at the same time (that is, the second gear is not cut off), causing interference between the first and second gears. In this case, the loader cannot move and brake.