Top sheets tucked in along the sides of the bed would still exert pressure on the upper surface of the feet, which may promote plantar flexion. The sides of top sheets, mitered at the foot of the bed, hang freely off the side of the bed. Making a vertical or horizontal toe pleat at the foot of the bed over the patients feet laves room for the feet to move freely and avoids exerting pressure on the upper surface of the feet, thus preventing plantar flexion.
The weight of the top sheets would still exert pressure on the upper surface of the feet, promoting plantar flexion. Trochanter rolls prevent external hip rotation, not plantar flexion.