"OnStart" for each PhysicsVelocity entity ?

What you need to do is to add:

• VelocityDelta & PhysicsVelocity components to an entity

OR

• VelocityDeltaComponent & PhysicsBody components to a GameObject (that you convert into an entity)

VelocityDeltaSystem looks for entities with VelocityDelta components then:

1. applies an one-time change in it's PhysicsVelocity.Linear

2. removes the VelocityDelta component (so the vel change won't reapeat)

   note: VelocityDeltaSystem executes only when the matching entities exist and is idle otherwise.

VelocityDeltaComponent.cs

 using UnityEngine;
 using Unity.Entities;
 using Unity.Jobs;
 using Unity.Mathematics;
 using Unity.Transforms;
 using Unity.Physics;
 
 [DisallowMultipleComponent]
 public class VelocityDeltaComponent : MonoBehaviour, IConvertGameObjectToEntity
 {
     [SerializeField] public Vector3 _delta = new Vector3{ z=10 };
     public void Convert ( Entity e , EntityManager cmd , GameObjectConversionSystem cs )
         => cmd.AddComponentData( e , new VelocityDelta{ Value = _delta } );
     #if UNITY_EDITOR
     void OnDrawGizmosSelected ()
     {
         Gizmos.color = Color.yellow;
         Gizmos.DrawLine( transform.position , transform.position + transform.TransformDirection(_delta) );
     }
     #endif
 }
 
 public struct VelocityDelta : IComponentData
 {
     public float3 Value;
 }
 
 public partial class VelocityDeltaSystem : SystemBase
 {
     EndFixedStepSimulationEntityCommandBufferSystem _commandBufferSystem;
     protected override void OnCreate ()
     {
         _commandBufferSystem = World.GetOrCreateSystem<EndFixedStepSimulationEntityCommandBufferSystem>();
     }
     protected override void OnUpdate ()
     {
         var cmd = _commandBufferSystem.CreateCommandBuffer().AsParallelWriter();
         
         Entities
             .ForEach( ( ref PhysicsVelocity velocity , in VelocityDelta delta , in LocalToWorld transform , in Entity e , in int nativeThreadIndex ) =>
             {
                 velocity.Linear += math.mul( delta.Value , math.inverse( (float3x3) transform.Value ) );
                 cmd.RemoveComponent<VelocityDelta>( nativeThreadIndex , e );
             } )
             .WithBurst()
             .ScheduleParallel();
         
         _commandBufferSystem.AddJobHandleForProducer( this.Dependency );
     }
 }