[ConnorMcLeod]

I think that proper way is to hook Ham_Player_Jump as post and there check if m_IdealActivity (or m_Activity) is equal to ACT_LEAP and frame (or gaitsequence) is equal to 0.

From HLSDK :

void CBasePlayer::Jump()
{
	Vector		vecWallCheckDir;// direction we're tracing a line to find a wall when walljumping
	Vector		vecAdjustedVelocity;
	Vector		vecSpot;
	TraceResult	tr;
	
	if (FBitSet(pev->flags, FL_WATERJUMP))
		return;
	
	if (pev->waterlevel >= 2)
	{
		return;
	}

	// jump velocity is sqrt( height * gravity * 2)

	// If this isn't the first frame pressing the jump button, break out.
	if ( !FBitSet( m_afButtonPressed, IN_JUMP ) )
		return;         // don't pogo stick

	if ( !(pev->flags & FL_ONGROUND) || !pev->groundentity )
	{
		return;
	}

// many features in this function use v_forward, so makevectors now.
	UTIL_MakeVectors (pev->angles);

	// ClearBits(pev->flags, FL_ONGROUND);		// don't stairwalk
	
	SetAnimation( PLAYER_JUMP );

	if ( m_fLongJump &&
		(pev->button & IN_DUCK) &&
		( pev->flDuckTime > 0 ) &&
		pev->velocity.Length() > 50 )
	{
		SetAnimation( PLAYER_SUPERJUMP );
	}

	// If you're standing on a conveyor, add it's velocity to yours (for momentum)
	entvars_t *pevGround = VARS(pev->groundentity);
	if ( pevGround && (pevGround->flags & FL_CONVEYOR) )
	{
		pev->velocity = pev->velocity + pev->basevelocity;
	}
}

// Set the activity based on an event or current state
void CBasePlayer::SetAnimation( PLAYER_ANIM playerAnim )
{
	/* [ ... ] */

	switch (m_IdealActivity)
	{
	case ACT_HOVER:
	case ACT_LEAP:
	case ACT_SWIM:
	case ACT_HOP:
	case ACT_DIESIMPLE:
	default:
		if ( m_Activity == m_IdealActivity)
			return;
		m_Activity = m_IdealActivity;

		animDesired = LookupActivity( m_Activity );
		// Already using the desired animation?
		if (pev->sequence == animDesired)
			return;

		pev->gaitsequence = 0;
		pev->sequence		= animDesired;
		pev->frame			= 0;
		ResetSequenceInfo( );
		return;

So in Jump hook post, you come right after SetAnimation( PLAYER_SUPERJUMP ).