A transmission assembly for a power extraction system, the transmission assembly comprising: an input shaft couplable to a low-pressure spool of a gas turbine engine whereby rotation of the low-pressure spool between first minimum and maximum speeds drives input shaft rotation at an input rotational speed; an output shaft couplable to downstream components whereby output shaft rotation at an output rotational speed drives rotation of the downstream components between second minimum and maximum speeds; two clutches (1331, 1332) by which the input shaft rotations at the input rotational speed drive output shaft rotations at the output rotational speed; and a controller configured to control openings and closings of the clutches according to an upshifting algorithm whereby a torque applied by the downstream components is used to reduce the output rotational speed to an output speed of a gear into which the transmission assembly is shifting by: closing a first one of the clutches in a first gear during a first ramping-up of the input rotational speed, opening the first one of the clutches at a first predefined output rotational speed, using the torque applied by the downstream components to reduce the output rotational speed to the output speed of a second gear, closing a second one of the clutches in the second gear when the output rotational speed reaches the output speed of the second gear and during a second ramping-up of the input rotational speed, opening the second one of the clutches at a second predefined output rotational speed and using the torque applied by the downstream components to reduce the output rotational speed to the output speed of a third gear, closing the first one of the clutches in the third gear when the output rotational speed reaches the output speed of the third gear and during a third ramping-up of the input rotational speed, opening the first clutch at a third predefined output rotational speed and using the torque applied by the downstream components to reduce the output rotational speed to the output speed of the fourth gear, closing the second one of the clutches in a fourth gear when the output rotational speed reached the output speed of the fourth gear. an input shaft couplable to a low-pressure spool of a gas turbine engine whereby rotation of the low-pressure spool between first minimum and maximum speeds drives input shaft rotation at an input rotational speed; an output shaft couplable to downstream components whereby output shaft rotation at an output rotational speed drives rotation of the downstream components between second minimum and maximum speeds; two clutches (1331, 1332) by which the input shaft rotations at the input rotational speed drive output shaft rotations at the output rotational speed; and a controller configured to control openings and closings of the clutches according to an upshifting algorithm whereby a torque applied by the downstream components is used to reduce the output rotational speed to an output speed of a gear into which the transmission assembly is shifting by: closing a first one of the clutches in a first gear during a first ramping-up of the input rotational speed, opening the first one of the clutches at a first predefined output rotational speed, using the torque applied by the downstream components to reduce the output rotational speed to the output speed of a second gear, closing a second one of the clutches in the second gear when the output rotational speed reaches the output speed of the second gear and during a second ramping-up of the input rotational speed, opening the second one of the clutches at a second predefined output rotational speed and using the torque applied by the downstream components to reduce the output rotational speed to the output speed of a third gear, closing the first one of the clutches in the third gear when the output rotational speed reaches the output speed of the third gear and during a third ramping-up of the input rotational speed, opening the first clutch at a third predefined output rotational speed and using the torque applied by the downstream components to reduce the output rotational speed to the output speed of the fourth gear, closing the second one of the clutches in a fourth gear when the output rotational speed reached the output speed of the fourth gear.