Dear Alberto Menegazzi,
for correct execution of Depth jump is important the muscular attitude during the flight phase (drop phase).
During the drop phase the legs and arms muscles should not be too hard and, at the same time, they should not be too relaxed.
Nevertheless, for the correct execution of depth jump it’s very important that athlete doesn’t think about it and has to be concentrated only on the sequent take-off phase.
The athlete should find the correct muscle attitude in natural way, through the practice and not through artificial way of the motor learning.