Around 70% of all tennis movement during a match is in the lateral direction
An object moving sideways creates a great deal of lateral momentum. In order to keep that momentum as a source of power, in combination with balance and power, the player needs to keep the inertia (lateral direction) without pulling the breaks before the stroke (deceleration should be performed during the stroke), transferring the linear momentum into the forward momentum.
In the moment of loading, the player slows down the momentum just enough to be able to transfer it forwards, keeping the body movement fluidity. If the movement is suddenly stopped applying the breaks to the motion with the whole foot stepping down, by the the Newton law of inertia, the upper body continues the movement sideways leaning over the base of support, losing the balance as the center of gravity will moves away from the base of support and player struggling not to fall over.
Hitting on the run
The key of the good stroke on the run is the timing of the backswing and timing of the loading step. The most important aspect of hitting on the run is keeping the balance while loading (power) and unloading (control). To insure the best balanced movement, players need to coordinate movement of the opposing sides of the lower and upper limbs (example, left arm and right leg) having them synchronized throughout their movement. This is a key element of the stroke on the run as enables the player to transfer linear to angular momentum by initiating a stroke from the leg drive (lower extremes for linear part of the forward momentum) and using the opposite side arm as the initiator of the trunk – shoulder rotation (angular momentum).
Expert players do not tend to get as close as possible to the ball while moving laterally trying to achieve as much balanced loading of the closer foot as a priority even if that means that the loading position is far from the future contact. No matter of the distance they use the loading foot to push as much as possible through the ball even if it means to hit the ball while having both feet above the ground.
As the closer foot is getting towards the final step before loading, the racquet arm is usually beginning the swing back away from the front arm, whose goal is to keep the front shoulder and front elbow prepared to accept the energy created from the loaded rear hip and leg and enable the forward momentum of the stroke. But if the loading step is too wide, the stroke mechanics will be unreliable as the base of support is too wide pressuring the player to chose stability over the joint mobility movement. This kind of body movement limits the stroke production as the hip immobility creates a breaking point of the efficient energy transfer forward.
On the other hand, maintaining the base of the support at the optimal size at the time of foot planting and loading, will enable the player to produce the forward movement of the hip and trunk through the ball creating the offensive stroke with penetrating capabilities.
Combination of the linear and angular movement are the foundation of the aggressive, offensive stroke. If the player feels that the situation and game constraints are limiting, then creating more accurate stroke (safety) rather then powerful one would be a wiser tactical choice.